God eternally exists as three persons, Father, Son, and Holy Spirit, and each person is fully God, and there is one God.
Within the one Being that is God, there exists eternally three coequal and coeternal persons, namely, the Father, the Son, and the Holy Spirit.
So God is one, but somehow also three. The fact is difficult to understand, but it is quite unavoidable in Scripture and central to the gospel. The doctrine of the Trinity attempts to account for this fact and to exclude heresies that have arisen on the subject. Its basic assertions are these: (1) God is one. (2) God is three. (3) The three persons are each fully God. (4) Each of the persons is distinct from the others. (5) The three persons are related to one another eternally as Father, Son, and Holy Spirit. –John Frame
Importance of Doctrine
For we must always remember that as we study this fact, we are not dealing with a doctrine about God, with an abstract concept, or with a scientific proposition about the nature of Divinity. We are not dealing with a human construction which we ourselves or which others have put upon the facts, and which we now try to analyze and logically to dismember. Rather, in treating of the Trinity, we are dealing with God Himself, with the one and true God, who has revealed Himself as such in His Word.
But to have the “right” answers about the Trinity, for example, and to actually believe in the reality of the Trinity, is all the difference in the world. The advantage of believing in the reality of the Trinity is not that we get an A from God for giving “the right answer.” Remember, to believe something is to act as if it is so. To believe that two plus two equals four is to behave accordingly when trying to find out how many dollars or apples are in the house. The advantage of believing it is that we can deal much more successfully with reality. Just try dealing with it as if two plus two equaled six.
Hence, the advantage of believing in the Trinity is that we then live as if the Trinity is real: as if the cosmos environing us actually is, beyond all else, a self-sufficing community of unspeakably magnificent personal beings of boundless love, knowledge, and power. And, thus believing, our lives naturally integrate themselves, through our actions, into the reality of such a universe, just as with two plus two equals four. In faith we rest ourselves upon the reality of the Trinity in action—and it graciously meets us. For it is there. And our lives are then enmeshed in the true world of God.
The doctrine of the Trinity was formulated by followers of Jesus Christ to safeguard the good news that in Jesus Christ we encounter God face to face. It was not devised to make God less understandable, or to make God so mysterious that the common people would have to depend on clergy and theologians to understand it for them as the JWs charge.
What then is lost from Christian theism, when we cease to think in trinitarian terms?... The conviction that God is love is the major casualty of unitarian theism.
The other losses of non-trinitarian religion are of a piece with this major casualty. Christ becomes one inspired man among others; and the Spirit a universal divine immanence within creation. We no longer have a living Saviour, by incorporation into whose Body we too can say ‘Abba, Father’; we no longer can think of our prayers and worship as taken up into the inner movement of God’s life.
Why was the church so concerned about the doctrine of the Trinity? Is it really essential to hold to the full deity of the Son and the Holy Spirit? Yes it is, for this teaching has implications for the very heart of the Christian faith. First, the atonement is at stake. If Jesus is merely a created being, and not fully God, then it is hard to see how he, a creature, could bear the full wrath of God against all of our sins. Could any creature, no matter how great, really save us? Second, justification by faith alone is threatened if we deny the full deity of the Son. (This is seen today in the teaching of the Jehovah’s Witnesses, who do not believe in justification by faith alone.) If Jesus is not fully God, we would rightly doubt whether we can really trust him to save us completely. Could we really depend on any creature fully for our salvation? Third, if Jesus is not infinite God, should we pray to him or worship him? Who but an infinite, omniscient God could hear and respond to all the prayers of all God’s people? And who but God himself is worthy of worship? Indeed, if Jesus is merely a creature, no matter how great, it would be idolatry to worship him—yet the New Testament commands us to do so (Phil. 2:9-11; Rev. 5:12-14). Fourth, if someone teaches that Christ was a created being but nonetheless one who saved us, then this teaching wrongly begins to attribute credit for salvation to a creature and not to God himself. But this wrongfully exalts the creature rather than the Creator, something Scripture never allows us to do. Fifth, the independence and personal nature of God are at stake: If there is no Trinity, then there were no interpersonal relationships within the being of God before creation, and, without personal relationships, it is difficult to see how God could be genuinely personal or be without the need for a creation to relate to. Sixth, the unity of the universe is at stake: If there is not perfect plurality and perfect unity in God himself, then we have no basis for thinking there can be any ultimate unity among the diverse elements of the universe either. Clearly, in the doctrine of the Trinity, the heart of the Christian faith is at stake. Herman Bavinck says that “Athanasius understood better than any of his contemporaries that Christianity stands or falls with the confession of the deity of Christ and of the Trinity.”Bavinck, The Doctrine of God p. 281. He adds, “In the confession of the Trinity throbs the heart of the Christian religion: every error results from, or upon deeper reflection may be traced to, a wrong view of this doctrine.”Ibid., p. 285.
Trinity in the Old Testament
The Old Testament may be likened to a chamber richly furnished but dimly lighted; the introduction of light brings into it nothing which was not in it before; but it brings out into clearer view much of what is in it but was only dimly or even not at all perceived before. The mystery of the Trinity is not revealed in the Old Testament; but the mystery of the Trinity underlies the Old Testament revelation, and here and there almost comes into view. Thus the Old Testament revelation of God is not corrected by the fuller revelation which follows it, but only perfected, extended and enlarged.
--B. B. Warfield
Trinity in NT as already fully developed
It is clear, in other words, that, as we read the New Testament, we are not witnessing the birth of a new conception of God. What we meet with in its pages is a firmly established conception of God underlying and giving it tone to the whole fabric. It is not in a text here and there that the New Testament bears its testimony to the doctrine of the Trinity. The whole book is Trinitarian to the core; all its teaching is built on the assumption of the Trinity; and its allusions to the Trinity are frequent, cursory, easy and confident…The doctrine of the Trinity does not appear in the New Testament in the making, but as already made.
--B. B. Warfield
The disciples were, indeed, “experiential Trinitarians.” They had walked with the Son, heard the Father speak from glory, and were now indwelt by the Holy Spirit. --James White
Problems with Modalism
i. The cross is emptied of its drama.
Rather, these Fathers clearly understood that what was at stake in this battle was the authenticity of God’s self-revelation. They understood that if the relationality found throughout the New Testament between the Father, Son, and the Holy Spirit was something that God merely assumed in time for the purpose of our salvation, as the modalists maintained, then even when we come to know God as he truly is. For what God is “truly” like, according to both ancient modalists and contemporary Oneness believers, lies in the supposed “undifferentiated Oneness,” which is hidden behind the three (or more) “masks” he wears in time.
In other words, in Oneness theology the three temporary “roles” of God do not arise out of God’s essential eternal being. God “plays” Father, Son, and Holy Spirit. But in his heart of hearts—whoever he is—he is not these three.
The big question, then, was whether the history of salvation was to be thought of as a sort of stage where God merely acts out certain roles that are otherwise foreign to his essential self, or whether this history reveals the innermost heart and internal depth of the eternal and infinite Godhead. Does God simply wear masks before us, or does he lovingly envelop us into the very depths of his essential and eternal being? The early Fathers, thankfully, perceived that the latter was the truth.
ii. Love between the Father and Son is compromised
Perhaps the most tragic implication of reducing the Father/Son personal distinction to a mere distinction of natures (or even outright “illusion”) is that it completely undermines the genuineness of the Father’s personal love for the Son and the Son’s personal love for the Father spoken of so poignantly throughout the New Testament. [David] Bernard specifically addresses the issue of this apparent loving relationship between the Father and the Son when he writes:
John 3:35, 5:20, and 15:9 state that the Father loves the Son [forget the numerous other texts!], and John 17:24 says the Father loved Jesus before the foundation of the world. In John 14:31 Jesus expresses love for the Father [again, one verse!]. All of these statements do not mean separate persons….What these verses express is the relationship between the two natures of Christ. The Spirit of Jesus loved the humanity and vice versa….Remember, the Son came to the world to show us how much God loves us and also to be our example. For these two objectives to be achieved, the Father and Son showed love for each other. [David Bernard, The Oneness of God: Series in Pentecostal Theology, vol. 1 (1983)]
Greg Boyd adds:
Therefore, in beholding the love of the Father for the Son and the love of the Son for the Father, we are not seeing anything that concerns the eternal heart of God himself. God could just as well have done without this pseudo-interpersonal “projected” love. Since the two “persons” are projected, and the love is expressed only for an example, none of this can tell us how God really is, how God really feels, what God is really like. In the Oneness view, we only know that God is “absolutely one,” so whatever he is like, he is not like he appears when the Father and Son are portrayed as distinct and as perfectly loving one another. And thus in the end the Incarnation shows us, not what God is (compare John 1:14, 18), not even what God is like, but only what we human beings should be like!
 Wayne Grudem Systematic Theology (Zondervan, 1994), 226.
 James White The Forgotten Trinity (Bethany House, 1998), 26.
 John Frame The Doctrine of God (Presbyterian and Reformed, 2002), 621-622.
 Herman Bavinck Our Reasonable Faith: A Survey of Christian Doctrine (Baker: 1956 [originally 1909]), 143.
 Dallas Willard The Divine Conspiracy (Harper Collins, 1998), 318.
 Robert M. Bowman Jr. Why You Should Believe in the Trinity: An Answer to Jehovah’s Witnesses (Grand Rapids: Baker, 1989), 138.
 Brian Hebblethwaite, “Recent British Theology,” One God in Trinity (Westchester: Cornerstone Books, 1980), 168-169.
 Wayne Grudem Systematic Theology (Zondervan, 1994), 247-248. Available online: https://www.biblicaltraining.org/library/trinity-wayne-grudem.
 Benjamin B. Warfield, “The Biblical Doctrine of the Trinity,” 141-142.
 Benjamin B. Warfield, “The Biblical Doctrine of the Trinity,”143.
 James White The Forgotten Trinity (Bethany House, 1998), 167-168.
 Gregory Boyd, Oneness Pentecostals and the Trinity (Baker, 1992), 178-179.
 Gregory Boyd, Oneness Pentecostals and the Trinity (Baker, 1992), 183-184.