The doctrine of the trinity is Scriptural.

*all passages are from ESVI will format my opening argument so that it is broken down into two parts: (1) defining the trinity and (2) proof texts. Defining the TrinityWhile I provided an elementary definition of the trinity in the description of this debate, I would like to flesh it out a little more and provide some clarity. I do not believe my opponent would contest the first part of my initial definition. Namely, there is one eternal God who is indivisible and infinite. He would disagree with the second portion. Namely, there are three persons that make up the one being of God. But it is important to emphasize the two “co” terms used. The three persons of the trinity are “co-equal” and “co-eternal.” That is, they are never identified with one another. They are identified as being fully deity, however. So, to emphasize, the Father is not the Son. The Son is not the Father. The Father is not the Spirit. The Spirit is not the Father. The Son is not the Spirit. The Spirit is not the Son. They are all equally God and all equally eternal. They are not identified as the same persons, however. They are merely identified as the same being. The best natural example of this would be an atom. An atom is made up of three components: protons, neutrons, and electrons. Consider the three components as persons. These three components, or persons, make up the one being, or atom. This is what the doctrine of the trinity teaches.Proof TextsNow that the groundwork has been laid, we can delve into Scripture. I will break this section down into three sub-sections: (1) the deity of the Son, (2) the deity of the Spirit, and (3) the triune formula. 1) The Deity of the SonLet’s dive into our first proof text for the deity of the Son:“In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through him, and without him was notany thing made that was made. […] And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. […] No one has ever seen God; the only God, who is at the Father's side, he has made him known.” (Jon. 1:1-3, 14, 18)This is perhaps one of the most famous texts for defending the deity of the Son and rightfully so. It is in this text that we are told the Word was in the beginning. The Word was with God. The Word was God. Not only this, but the Word created all things. And there is nothing that was made without Him.1a) The BeginningWhat does the phrase “in the beginning” refer to? Could it be in reference to the same “beginning” mentioned in Genesis 1:1? John was very careful as to which form of “was” (en) he used. The form of the verb “was” that John used indicates a timelessness. [1] That is, as far back as we push our understanding of “the beginning,” the Word was there. Namely, the Word is eternal, having no beginning.Though this is not the only time that John uses this form of the verb “was” in his prolog. The fact of the matter is that he uses it in reference to the Word every single time. He is careful to not use this form when referencing others (he uses egeneto (“to become”) for others, see verses 6, 10, and 12). It is only in verse 14 that we see John using egeneto in regard to the Word. This is, of course, in reference to when the Word becomes flesh. 1b) With GodWhat are we to think of “the Word was with God”? Well, it should be noted that the term “was” is in the exact same form as in John 1:1a. Though John also uses the Greek preposition pros. Pros means “equivalent to.” [2] So, the Son is “equivalent to” God. Is John suggesting that there are two eternal and equivalent Gods? This is cleared up in light of John 1:1c.1c) Was GodJohn, again, is using the same verb form for “was.” John is not trying to distinguish the Word and Father as two different gods. Nor is he trying to say that they are the same. Rather, he is taking steps to define the triune Godhead. John distinguished the two while telling us that they have eternally co-existed, they are co-eternal.I believe that should be sufficient enough a discussion of John’s prolog. I do not doubt that it will resurface again later in this debate. The next proof text that I would like to turn to is Thomas’ confession:“Thomas answered him, ‘My Lord and my God!’ Jesus said to him, ‘Have you believed because you have seen me? Blessed are those who have not seen and yet have believed.’” (Jon. 20:28, 29)This passage holds quite a bit of significance and it is on quite an elementary level. Thomas declares that Jesus is his Lord (Kyrios) and his God (ho Theos). The same terms used of the Father. What is Jesus’ response? He does not rebuke him. Rather, Jesus says “blessed are those” who believe as you do, yet do not need the visual evidence. Jesus blesses those who hold the same conviction as Thomas. This testimony that Jesus is God is quite clearly reinforced by Paul:“To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen.” (Rom. 9:5)Now, I believe it is key to dive into some deeper texts to verify the deity of the Son. We will start in the book of Hebrews. In the first chapter, we are told that the Son is “the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power.” We are told that “all God's angels worship him.” What does the one on the throne say of His Son? “Your throne, O God, is forever and ever, the scepter of uprightness is the scepter of your kingdom. You have loved righteousness and hated wickedness; therefore God, your God, has anointed you with the oil of gladness beyond your companions.” The one on the throne also says: “[the Son] laid the foundation of the earth in the beginning, and the heavens are the work of your hands; they will perish, but you remain; they will all wear out like a garment, like a robe you will roll them up, like a garment they will be changed. But you are the same, and your years will have no end.” I do not believe a whole lot has to be said here. The descriptions that were given to Jehovah God in the Old Testament are now being said of the Son by the One on the throne. Jesus is said to be the “true God” and “eternal life” in 1 John 5:20. Jesus is said to be the “great God” and “Savior” in Titus 2:13 and 2 Peter 1:1. There are numerous passages that I could cite, but I do not wish to exhaust my space. I will list one more passage to show the deity and eternal nature of Christ:“And I heard every creature in heaven and on earth and under the earth and in the sea, and all that is in them, saying, ‘To him who sits on the throne and to the Lamb be blessing and honor and glory and might forever and ever!’ And the four living creatures said, ‘Amen!’ and the elders fell down and worshiped.” (Rev. 5:13, 14)The Lamb is distinguished from all other creatures. He is, therefore, not a creature. He has to be eternal. He also receives the same blessings, honor, glory, and worship as the One on the throne.2) The Deity of the SpiritPerhaps it would be useful to actually identify the Spirit as a person before we can begin discussing the deity of Him. Some passages to consider are:“But when the Helper comes, whom I will send to you from the Father, the Spirit of truth, who proceeds from the Father, he will bear witness about me.” (given a masculine pronoun; Jon. 15:26)“And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.” (one can grieve the Holy Spirit; Eph. 4:30)“But Peter said, ‘Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land?’” (one can lie to the Holy Spirit; Act. 5:3)“When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come.” (the Holy Spirit can teach; Jon. 16:13)“All these are empowered by one and the same Spirit, who apportions to each one individually as he wills.” (the Spirit has His own will; 1 Cor. 2:10, 11)I believe that those above passages should be sufficient in proving that the Holy Spirit is indeed a person. Though how do we know that the Spirit is deity? Peter himself identifies the Holy Spirit as God. Going back to Acts 5, Peter states that Ananias lied “to God,” but we are told that Ananias was lying to the Holy Spirt. In other words, there is no distinguishing between God and the Holy Spirit. Paul also does this same thing in 2 Corinthians 3. He states that “the Lord is the Spirit.”3) The Triune FormulaDoes the Bible ever state the triune formula explicitly? See for yourselves:“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...” (Mat. 28:19)“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)Other passages to consider are: Romans 15:16, 30; 2 Corinthians 1:21-22; Ephesians 2:18.ConclusionWe have, after this round, established a clear definition for the trinity and provided the principle proof texts for the deity of the Son, the deity of the Spirit, and the triune formula. I would like to conclude by saying that the Gospel itself is Trinitarian. It flows from the decree of the Father. It is accomplished in the perfect work of the Son: incarnation, perfect life in obedience to the Father, death, burial, resurrection, and ascension. Now at the right hand of the Father as an intercessor or mediator, he is one work representing those who are united with him. And from the Holy Spirit, who comes and makes these things real in our lives, in our hearts, at the time that God decrees to do so.Sources:[1] Harris, Jesus as God, p. 54[2] http://biblehub.com...