The following are questions with answers taken from various Wikipedia articles. This is to highlight some relevant points on the subject—"Scriptural Basis Of The Trinity Doctrine"—which people should understand before continuing the bible study. The answers are excerpt taken from the full articles written on the subject. You can readily study the full articles written on the subject by clicking on the source link after each answer.
Note: The answers to the following religious questions are taken from various Wikipedia articles due to their neutral point of view principle.  http://en.wikipedia.org/wiki/Wikipedia:Neutral_point_of_view  However, it is still highly recommend that you search the internet for more information on the subject.

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From the preceding lessons, we've learned that the the Old Testament or the Hebrew Bible does not contain the doctrine of the Trinity. (Lesson-2C.2) And that the New Testament does not use the word Trinity nor explicitly teach it. (Lesson-2C.3) The doctrine is not explicitly stated in the New Testament because "no New Testament writer" ever expounded on the relationship among the three (the Father, the Son and the Holy Spirit) in the detail (Lesson-3C.4). But Trinitarians claim that the Bible provides the material upon which the doctrine of the Trinity is based. Let us continue to study this popular belief so we may come to know the truth behind it.

Verses upon which the Doctrine of the Trinity is based

2D.1   What are the biblical verses that are claimed to provide material upon which the doctrine of the Trinity is based?

References used from Scripture

The New Testament does not use the word "Τριάς" (Trinity) nor explicitly teach it, but provides the material upon which the doctrine of the Trinity is based.^[22] It required reflection by the earliest Christians on the coming of Jesus and of what they believed to be the presence and power of God among them, which they called the Holy Spirit; and it associated the Father, Son, and Holy Spirit in such passages as the Great Commission: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit"^{[Matt. 28:19]} and Paul the Apostle's blessing: "The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all,"^{[2 Cor. 13:14]} while at the same time not contradicting the Jewish Shema Yisrael: "Hear, O Israel: the Lord our God, the Lord is one."^{[Deuteronomy 6:4][1]} Apart from the passages that speak of Father Son and Holy Spirit, there are many passages that refer to God and Jesus without also referring to the Spirit.^[23]

According to Christian tradition the Trinity was introduced by the Gospels and Jesus Christ himself^[24] "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you."^{[Matt. 28:19-20]} Jesus thus mentions the Father, the Son, and the Holy Spirit in a phrase that may suggest that there is one name that encompasses all three.

The Old Testament refers to God's word,^[25] his spirit,^[26] and Wisdom.^[27] These have been interpreted as foreshadowings of the doctrine of the Trinity,^[28] as have been also narratives such as the appearance of the three men to Abraham.^{[Gen. 18][6]} Some Church Fathers believed that a knowledge of the mystery was granted to the prophets and saints of the Old Dispensation, and that they identified the divine messenger of Genesis 16:7, 21:17, 31:11, Exodus 3:2 and Wisdom of the sapiential books with the Son, and "the spirit of the Lord" with the Holy Spirit.^[29]

However, it is generally agreed that it would go beyond the intention and spirit of the Old Testament to correlate these notions directly with later Trinitarian doctrine.^[29][30]

The Gospel of John opens by declaring, as usually translated: "In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made." The rest of John 1 makes it clear that "the Word" refers to Jesus Christ. Thus John introduces a seemingly impossible contradiction, that Jesus both "was with God" and "was God" at the same time, and that was true from the beginning of creation. John also portrays Jesus Christ as the creator of the Universe, such that "without him nothing was made that   has been made."^[John 1:3]

The Apostle John is identified as the "one whom Jesus loved" thus perhaps being the closest Apostle to Jesus. Jesus also instructed John to adopt Jesus' mother Mary as John's own in Mary's old age^[Jn 19:26] such that John would have had the entire knowledge of Jesus' family when writing his Gospel. Some scholars question this, however, as the gospel of John is believed to have been written no earlier than the last decade of the first century (ca. 96 CE, according to Catholic tradition).^[31]

Jesus frequently referred to the "Father" as God as distinct from himself, but also discussed "The Holy Spirit" as a being distinct from either God the Father or Jesus himself.

These things I have spoken to you while abiding with you. But the Helper, the Holy Spirit, whom the Father will send in My name, he will teach you all things, and bring to your remembrance all that I said to you. – John 14:25-26

In this passage, Jesus portrays the Father sending the Holy Spirit—that is the Father and the Holy Spirit are two distinctly different persons, and portrays both the Father and the Holy Spirit as distinct from Jesus himself. Thus even apart from whether Jesus was God, Jesus declares that the Father and the Holy Spirit are two different persons, both of them divine. In the same way, the Old Testament frequently refers to "the Spirit of God" as something slightly different from God himself.

The fourth Gospel also elaborates on the role of Holy Spirit, sent as an advocate for believers.^[32] The immediate context of these verses was providing "assurance of the presence and power of God both in the ministry of Jesus and the ongoing life of the community"; but, beyond this immediate context, these verses raised questions of relationship between Father, Son and the Holy Spirit, especially as concerns their distinction and their unity. These questions were hotly debated over the ensuing centuries, and mainstream Christianity resolved the issues by drawing up creeds.^[32]

However, some scholars dispute the authenticity of the Trinity and argue that the doctrine is the result of "later theological interpretations of Christ's nature and function."^[33][34] The concept was expressed in early writings from the beginning of the second century forward. Some believe the concept was introduced in the Old Testament book of Isaiah written around 700 years before Jesus, copies of which were preserved from 300 years before Jesus in the Dead Sea Scrolls. Isaiah 9:6 prophesies "For unto us a Child is born, Unto us a Son is given; And the government will be upon his shoulder. And his name will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace." Thus a son who will be born at a particular point in history (to a virgin or young woman^[Isa. 7:14] is also "Mighty God, Everlasting Father". This is the Christian teaching that God exists simultaneously as the Eternal God and also as a Son (Jesus) born to a virgin. Isaiah refers to the Son as "Mighty God, Everlasting Father".

Various passages from both the Christian and Hebrew scriptures have been cited as supporting this doctrine, while other passages are cited as opposing it.
Source: Trinity, http://en.wikipedia.org/w/index.php?title=Trinity&oldid=340760472 (last visited Jan. 29, 2010).

As explained above, the New Testament does not use the word "Trinity" nor explicitly teach it, but it is claimed to provide the material upon which the doctrine of the Trinity is based.  It is also claimed that according to Christian tradition the Trinity was introduced by the Gospels and Jesus Christ himself.

The following verses are claimed to provide the basis of the Trinity doctrine.  Please check/study these verses if said verses really provide basis of the Trinity doctrine. 

• Matthew 28:19 - "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, (NKJV)

• 2 Corinthians 13:14 --  The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Spirit be with you all. Amen. (NKJV)

• John 1:1 &3 -   1. In the beginning was the Word, and the Word was with God, and the Word was God.  3. All things were made through Him, and without Him nothing was made that was made. (NKJV)

• John 24:25-26 -  "These things I have spoken to you while being present with you.  "But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all things that I said to you. (NKJV)

• Isaiah 9:6 -  For unto us a Child is born, unto us a Son is given; and the government will be upon His shoulder. And His name will be called Wonderful, Counselor, Mighty God, Everlasting Father, Prince of Peace. (NKJV)

Study suggestions: Use other versions of the Bible in your study. Search the internet for more information on the subject and/or the explanation of the above verses. Search and study other supporting verses that may not be included in the above list.

Verses cited to imply support of the Trinity Doctrine

2D.2   What biblical verses are cited to imply support of the Trinity doctrine?

Scriptural texts cited as implying support

The diverse references to God, Jesus, and the Spirit found in the New Testament were later systematized into the idea of a Trinity—one God subsisting in three persons and one substance—in order to combat heretical tendencies of how the three are related and to defend the church against charges of worshiping two or three gods.^[32] The doctrine itself was not explicitly stated in the New Testament and no New Testament writer expounds on the relationship among the three in the detail that later writers do. Thus, while Matthew records a special connection between God the Father and Jesus the Son,^{[Matt. 11:27]} he falls short of claiming that Jesus is equal with God^{[Matt. 24:36][32]} although John is more explicit and writes that Jesus Christ told the Jews: "I and the Father are one".^{[John 10:30]}

The most influential New Testament text was the reference to the three Persons in the baptismal formula in Matthew 28:19. Other passages also were seen as having Trinitarian overtones, such as the Pauline benediction of 2 Cor. 13:14.^[6]

The Gospel of John starts, as generally understood and translated,^[36] with the affirmation that in the beginning Jesus as Word "was with God and ...was God",^[John 1:1] and ends with Thomas's confession of faith to Jesus, "My Lord and my God!"^{[John 20:28][32]} There is no significant tendency among modern scholars to deny that these two verses identify Jesus with God.^[37] The same Gospel also suggests that Jesus' use of the term "Son of God" inferred essential equality and unity of Father and Son—"…making himself equal to the Father"^{[John 5:18] [19:7]} and saying "I and the Father are one."^[10:30] John also suggests a hierarchy when Jesus is quoted as saying, "The Father is greater than I,"^[14:28] a statement appealed to by Marcionism, Valentinianism, Arianism who held non-trinitarian views.^{[citation needed]}

Summarizing the role of scripture in the formation of Trinitarian belief, Gregory Nazianzen argues in his Orations that the revelation was intentionally gradual:
The Old Testament proclaimed the Father openly, and the Son more obscurely. The New manifested the Son, and suggested the deity of the Spirit. Now the Spirit himself dwells among us, and supplies us with a clearer demonstration of himself. For it was not safe, when the Godhead of the Father was not yet acknowledged, plainly to proclaim the Son; nor when that of the Son was not yet received to burden us further.^[38]
Source: Trinity, http://en.wikipedia.org/w/index.php?title=Trinity&oldid=340760472 (last visited Jan. 29, 2010).

The diverse references to God, Jesus, and the Spirit found in the New Testament were later systematized into the idea of a Trinity—one God subsisting in three persons and one substance. 

Based on the above references, the following verses are cited to imply support the doctrine of the Trinity. Please check/study these verses if said verses really support and/or confirm the Trinity doctrine.

• John 10:30 -  "I and My Father are one.'' (NKJV)
  
  
• John 20:28 - And Thomas answered and said to Him, "My Lord and my God!' (NKJV)
  
  
• John 5:18 - Therefore the Jews sought all the more to kill Him, because He not only broke the Sabbath, but also said that God was His Father, making Himself equal with God.(NKJV)
  
  
• John 19:7 - The Jews answered him, "We have a law, and according to our law He ought to die, because He made Himself the Son of God.'' (NKJV)

Study suggestions: Use other versions of the Bible in your study. Search the internet for more information on the subject and/or the explanation of the above verses. Search and study other supporting verses that may not be included in the above list.

Verses that directly reference the Father, Son, and Holy Spirit at the same time

2D.3   What are the verses that are claimed to support the Trinity doctrine because these verses directly reference the Father, Son, and the Holy Spirit at the same time?

References to Father, Son, and Holy Spirit 

A few verses directly reference the Father, Son, and Holy Spirit at the same time:

• "As soon as Jesus Christ was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and landing on him. And a voice from heaven said, 'This is my Son, whom I love; with him I am well pleased.'"^{[Matt. 3:16–17] [Mark 1:10–11] [Luke 3:22] [John 1:32]}
• "The angel answered and said to her, 'The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy Child shall be called the Son of God.'"^[Luke 1:35]
• "How much more, then, will the blood of Christ, who through the eternal Spirit offered himself unblemished to God, cleanse our consciences from acts that lead to death, so that we may serve the living God!"^[Heb. 9:14]
• "But Stephen, full of the Holy Spirit, looked up to heaven and saw the glory of God, and Jesus standing at the right hand of God." ^[Acts 7:55]
• This passage contains many complex formulations of the relationship between God, Christ, and Spirit, including "the Spirit of him who raised Jesus from the dead,"^[Rom. 8:11] "all who are led by the Spirit of God are sons of God,"^[8:14-17] and "the Spirit intercedes for the saints according to the will of God."^[8:26-27]

Some even reference these as part of a single formula:

• "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit"^{[Matt. 28:19]} (see Trinitarian formula). It has been claimed that writings of Eusebius show the mention of Father, Son, and Holy Spirit to have displaced a request by Jesus that his disciples baptize people in his name,^[39] but all manuscripts of the Gospel of Matthew contain, without any variation, the mention of the Trinity.^[40]
• "The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with all of you."^{[2 Cor. 13:14]}

Comma Johanneum

Main article: Comma Johanneum

In addition to these, 1 John 5:7, which is found in the King James Version but not in modern English translations nor in the official Latin text (a revision of the Vulgate) of the Roman Catholic Church,^[41] states: "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." However, this Comma Johanneum is not considered to be part of the genuine text.^[42] It is commonly found in Latin manuscripts, but is absent from the Greek manuscripts, except for a few late examples, where the passage appears to have been back-translated from the Latin. Erasmus, the compiler of the Textus Receptus, on which the King James Version was based, noticed that the passage was not found in any of the Greek manuscripts at his disposal and refused to include it until presented with an example containing it, which he rightly suspected^{[neutrality is disputed]} was a gloss after the fact.^[43] Although the Latin Church Father, Saint Cyprian, is thought to have referred to the passage,^[44] it is now considered not to have been part of the original text.
Source: Trinity, http://en.wikipedia.org/w/index.php?title=Trinity&oldid=340760472 (last visited Jan. 29, 2010).

From the above information, the following verses are cited to directly reference the Father, Son, and Holy Spirit at the same time.

Please check/study the following verses to determine if said verses really mean to support and confirm the Trinity doctrine by directly referencing the Father, Son, and Holy Spirit at the same time.

•  Matthew 3:16-17 , Mark 1:10-11,  Luke 3:22,  John 1:32
  • Matthew 3:16-17 - Then Jesus, when He had been baptized, came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. 17. And suddenly a voice came from heaven, saying, "This is My beloved Son, in whom I am well pleased.'' (NKJV)
  • Mark 1:10-11 - And immediately, coming up from the water, He saw the heavens parting and the Spirit descending upon Him like a dove. 11. Then a voice came from heaven, "You are My beloved Son, in whom I am well pleased.'' (NKJV)
  • Luke 3:22 - And the Holy Spirit descended in bodily form like a dove upon Him, and a voice came from heaven which said, "You are My beloved Son; in You I am well pleased.'' (NKJV)
  • John 1:32 -  And John bore witness, saying, "I saw the Spirit descending from heaven like a dove, and He remained upon Him. (NKJV)
     
•  Luke 1:35 - And the angel answered and said to her, "The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that Holy One who is to be born will be called the Son of God. (NKJV)
•  Hebrew 9:14 - how much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, purge your conscience from dead works to serve the living God? (NKJV)
•  Acts 7:55 - But he, being full of the Holy Spirit, gazed into heaven and saw the glory of God, and Jesus standing at the right hand of God, (NKJV)
•  Romans 8:11, Romans 8: 14-17,  Romans 8:26-27
  • Romans 8:11 - But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ from the dead will also give life to your mortal bodies through His Spirit who dwells in you. (NKJV)
  • Romans 8:14-17 - 14. For as many as are led by the Spirit of God, these are sons of God.
    15. For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, "Abba, Father.''
    16. The Spirit Himself bears witness with our spirit that we are children of God,
    17. and if children, then heirs heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together. (NKJV)
  • Romans 8:26-27 - 26. Likewise the Spirit also helps in our weaknesses. For we do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us with groanings which cannot be uttered. 27. Now He who searches the hearts knows what the mind of the Spirit is, because He makes intercession for the saints according to the will of God. (NKJV)
•  1 John 5:7 - For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (KJV) -
  • Note: 1 John 5:7, which is found in the King James Version but not in modern English translations nor in the official Latin text (a revision of the Vulgate) of the Roman Catholic Church. For more details, see above references on Comma Johanneum.

Study suggestions: Use other versions of the Bible in your study. Search the internet for more information on the subject and/or the explanation of the above verses. Search and study other supporting verses that may not be included in the above list.

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Why the Trinity doctrine is a mystery

After studying the scriptural basis of the Trinity Doctrine, many people have found out that it is indeed difficult to comprehend. In fact many found it impossible to understand. Let us continue our study so we may come to know why the Trinity doctrine is a mystery.

2D.4   What do some people say regarding their understanding of the doctrine of the Trinity?

The following are some comments of people who have studied the doctrine of the Trinity:

• The Trinity is a wonderful mystery. No one understands it. The most learned theologian, the holiest Pope, the greatest saint, all are mystified by it as a child of seven.
  [Martin J. Scott, S.J., God and Myself, Nhil Obstat: Arthurus J. Scanlan, S.T.D., Imprimatur: Joannes Cardinalis Farley (P.J. Kenedy and Sons, 1917), pp. 118-119.]
• Trinitarians say that "the doctrine of the Trinity is [...] a deep mystery that cannot be fathomed by the finite mind."^[7]
  Source: Wikipedia, Nontrinitarianism, http://en.wikipedia.org/wiki/Nontrinitarianism (as of June 17, 2010, 15:11 GMT).
• The doctrine of the Trinity — that God the Father, God the Son, and God the Holy Spirit are each equally and eternally the one true God — is admittedly difficult to comprehend, and yet is the very foundation of Christian truth. Although skeptics may ridicule it as a mathematical impossibility, it is nevertheless a basic doctrine of Scripture as well as profoundly realistic in both universal experience and in the scientific understanding of the cosmos.
  Authors: Henry Morris and Martin Clark (excerpted from The Bible Has the Answer by Morris and Clark, published by Master Books, 1987).
• "The mind of man cannot fully understand the mystery of the Trinity. He who would try to understand the mystery fully will lose his mind. But he who would deny the Trinity would lose his soul"
  (Harold Lindsey and Charles J. Woodbridge, A handbook of Christian truth, pp 51-52).

Trinitarians and other authors commented that the doctrine of the Trinity is . . . . . .

• The Trinity is a wonderful mystery. No one understands it. The most learned theologian, the holiest Pope, the greatest saint, all are mystified by it as a child of seven.
• It is a deep mystery that cannot be fathomed by the finite mind
• It is admittedly difficult to comprehend.
• Skeptics ridicule it as a mathematical impossibility.
• The mind of man cannot fully understand the mystery of the Trinity. He who would try to understand the mystery fully will lose his mind.

2D.5   Why is the doctrine of the Trinity a mystery?

The doctrine of the Trinity — that God the Father, God the Son, and God the Holy Spirit are each equally and eternally the one true God — is admittedly difficult to comprehend. It is said that the "Trinity is a mystery no one understands". 

• For how indeed could anyone understand the teaching that the three persons, each of whom is a true God, are only one God?
• In other words, how could anyone understand the teaching that one plus one plus one are equal to one?

John Walsh a Jesuit priest has this to say:

God, of course, can not perform an absurdity, a contradiction in terms. He cannot for instance, make two and two equal five.
[John Walsh, This is Catholicism (New York: Image Books, 1959) p. 25.]

To say that "one plus one plus one are equal to one" is no better that saying "two plus two equal five". It is an absurdity says Walsh, a contradiction in terms.

Another Jesuit priest  C.F. Blount states in his book The Blessed Trinity:

"the dogma of the Blessed Trinity is a mystery in the fullest sense" . . . . "it cannot be proved by reason, . . . nay, it cannot be even be proved to be possible"
[Rev. C.F. Blount, S.J., The Blessed Trinity (London: Catholic Truth Society), p.2.]

Thus, it is clearly accepted by Trinitarians that the doctrine of the Trinity is a mystery because . . . .

• it cannot be proved by reason
• it cannot even be proved to be possible.

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FOR MORE INFORMATION ON THE SUBJECT:
Many articles have been written about the Trinity doctrine and many are available in the internet. Please search the internet using the search phrase - scriptural proof of the Trinity doctrine.