The belief in one unique God, Creator of heaven and earth, who exists as Father, Son and Holy Spirit, is central to mainstream Christian faith. The core biblical truths which this doctrine encapsulates, that there is one God and yet Father, Son and Holy Spirit are each personal and divine in perfect unity, have been believed since the earliest days of the church when the Apostles were still alive and teaching.
The word “Trinity,” which means tri-unity or “three-in-oneness” was used from the early third century and simply encapsulates these biblical concepts. Yet people often shy away from grappling with the doctrine. It may seem too complex for “ordinary” people to understand, as if years of Bible College are required to grasp its intricacies. Some may mistakenly believe that it should be accepted on blind faith, as if it were somehow irreverent to seek to explain or understand the nature of God, or that to do so exhibits a lack of faith in divine “mysteries.” Others, particularly those antagonistic to the doctrine, argue that it is irrational and its “non-scriptural” terminology somehow proves that it cannot be a biblical doctrine. Each of these positions misses the point. Jesus Christ came to reveal God in a more detailed, intimate way than ever before, so that we could be in relationship with him through eternity. Jesus said,
I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me. Father, I desire that they also, whom you have given me, may be with me where I am, to see my glory that you have given me because you loved me before the foundation of the world. O righteous Father, even though the world does not know you, I know you, and these know that you have sent me. I made known to them your name, and I will continue to make it known, that the love with which you have loved me may be in them, and I in them.” (John 17:20–26)
God wants his children to know him and to be one with him, but at the same time he appreciates that we are limited human beings. It would be presumptuous to assume we can know God to the ultimate detail, for we can only know him to the extent he has chosen to reveal himself, most fully in Christ (Heb 1:1–2; Col 2:9; John 14:7–9). For God’s ways are infinitely higher than ours (Isa 55:8–9). In the Old Testament, we receive glimpses of the Father, Son and Holy Spirit in ways which suddenly come into focus in the New. We see that Jesus shares the attributes, honour and glory of God, works the works of God and shares the prerogatives of God. Motifs such as Shepherd, Judge, Saviour, Lord which apply to God are seen to have been referencing Christ all along. The Holy Spirit, acting so powerfully in the Old Testament, becomes the sanctifying Comforter in the New. The mighty God of Israel, who at times seemed so distant because of his Holiness in the face of his wayward people’s sins (Isa 59:2) is revealed as the loving Father of his Son and, through union with Christ, of all who are saved.
Yet all through Scripture we find the unwavering testimony that there is one God, who alone is to be worshipped (Deut 6:4–5; Isa 45:5; Rev 4:11). The first point of misunderstanding, that the Trinity is perceived by some to teach three Gods. That is absolutely not the case, and such a doctrine of Tritheism has never been the belief of the Christian church. The Bible is adamantly monotheistic, and so are mainstream Christians. The basis of monotheism is that there are fundamentally two types of reality; God and everything else. Everything else was created by God. Only God is self-existent, all powerful, intrinsically holy and good. Only God sustains and controls his creation, only God is sovereign, only God is worthy of worship. God sits above a fundamental dividing line between Creator and created. Absolutely everything else that ever has, does or will exist lies below that line. This above-the-line God does not and will not share his glory with another; he alone is God. To worship anyone or anything else is to worship his creation and that is idolatry (Isa 42:5–8; 44:6; Matt 4:10). In the Old Testament, as we look up above that line to the one and only God, it’s like seeing a single bright light, a magnificent, incomprehensible glory whose character and deeds are gradually revealed through salvation history
With the coming of Jesus, God is suddenly revealed in a great deal more detail. It is as if that incomprehensible light comes into focus, and we see God to exist and to act as three distinctives. To use two very limited analogies, it’s like looking at white light through a prism and discovering it is made up of different glorious colours, or like observing a point of light in the dark sky which we assumed to be a single star and then through a telescope realising it is actually a galaxy. These are two crude analogies from the created world to illustrate that unity can contain multiplicity, but that this complexity needs to be revealed. The New Testament brings that light into focus and we see that Jesus Christ the Son of God makes claims to deity, does the works of God, shares the attributes and prerogatives of God and is worthy of the honour, glory and worship due to God alone. We also see the Holy Spirit revealed as a divine personality who indwells believers as Comforter, Advocate, Sanctifier and distributor of divine gifts.
This “Trinity in unity” is not a contradiction, even though it has been misunderstood in various ways. Some people have been so concerned to uphold the numerical oneness of God that they blur the clear distinctions between the Father, Son and Spirit, suggesting that the one God acts in these three roles or modes successively; this error was called modalism. Others seek to uphold the unity of God by denying the deity of the Son and Holy Spirit; the Son becomes a sort of hybrid being, a first creation or an adopted and glorified human and the Holy Spirit an impersonal power. Others have emphasised the distinctiveness of Father, Son and Spirit to the point where the unity of the Godhead seems imperilled. The unity of the Godhead surpasses all concepts of unity in creation, even in those intimate relationships which are modelled on the unity of God, such as marriage and union of the church under Christ.
Another mistake is to see the Trinity in mathematical terms, arguing through rationalistic reasoning., “How can one plus one plus one equal three? That’s absurd!” Inevitably, such arguments are accompanied by a misrepresentation of the doctrine, such as “How can there be three persons in one person?” But it’s not a case of finding a more satisfactory mathematical model, such as 1 x 1 x 1 = 1, because the Trinity is not about mathematics. Here’s what Roger Nicole had to say:
It is important to recognise that the doctrine of the Trinity is a mystery. It is not, however, an absurdity, as some people have viewed it. Specifically, it is not asserted that God is one in the same respect in which he is three. What is propounded is that there is unity of essence, that this one essence is shared alike by each of the three persons, and that the three are conjoined in a total harmony of will and being, which far surpasses the unity observed between distinct individuals in humanity.” 
Above “The Line” that separates Creator from creation, Deity from everything else, we have Father, Son and Holy Spirit, one God in perfect unity, completely separate from everything that is not Deity. The sense in which the Godhead is one is the essence or “substance” of divinity, what it means to be God, immortal, omniscient, incorruptible, the God who is love. Richard Bauckham calls it the “unique divine identity.” The early church used the term ousia, meaning essence or substance, something which exists. God has one ousia or substance and this is the sense in which he is God and no one and nothing else shares the divine ousia. This divine ousia exists in three distinctives which the early church called hypostaseis (Greek) or personae (Latin) translated “Persons.” Words change their meaning over time, and the word “person” probably carries too much of an individualistic sense in modern English and perhaps if we were writing the creeds today we would choose a different word to express the distinctiveness of Father Son and Spirit, even as they share the divine ousia in perfect unity. Each hypostasis or Person is the ousia of God distinguished by his own properties, roles and characteristics as revealed with the coming of the Word. The Father sent the Son (John 12:49; Gal 4:4), who humbled himself taking the form of a servant (Phil 2:6–9) and was submissive to the Father in order to accomplish his life of perfect obedience to death. Now the Son has returned to the Father, the Father and Son have sent the Holy Spirit (John 14:26; 15:26). John Calvin expressed it this way:
It is not fitting to suppress the distinction that we observe to be expressed in Scripture. It is this: to the Father is attributed the beginning of activity, and the fountain and wellspring of all things; to the Son, wisdom, counsel and the ordered disposition of all things; but to the Spirit is assigned the power and efficacy of that activity.” 
These divine relationships are eternal. The Son did not peel away from the Godhead and exist for the first time at the incarnation, nor did the Spirit suddenly become personal when sent as the Other Comforter. The man Jesus Christ began to exist for the first time at his conception (Matt 1:20; Luke 1:35), the point of the in-fleshing (incarnation) of the Word (John 1:1, 2, 14) but the Son had been in eternal relationship with the Father and the Spirit (John 16:28; 17:5,24). God has existed eternally in this relationship of love and also interacts with his creation as Father, Son and Holy Spirit, each distinctive and yet perfectly united. The way we experience God through creation and salvation is consistent with Who he is eternally (Matt 28:19; 2 Cor 13:14). But ultimately, it is not these distinctive acts such as sending, dying on the cross, indwelling and sanctifying that distinguish the three, but their eternal relationships. All their unified acts are the work of the unified Godhead (1 Cor 12:4–6; Eph 4:4–6). The God who is known to us through his revelation in Creation, his written Word, his Son and his indwelling Spirit corresponds to the way God actually is in his essential nature. To experience God’s salvation, to be in relationship with him, is to experience God’s life. “And this is eternal life,” said Jesus, “that they know you the only true God, and Jesus Christ whom you have sent” (John 17:3).
The Trinity is in one sense a difficult concept, for like much to do with God we don’t see him displayed this way physically before us. Ultimately we need to humbly acknowledge that God is beyond our comprehension, but he has graciously revealed himself to the appropriate extent. In the future, we will be blessed to see him and know him more intimately in all his glory. We cannot yet understand how God has existed for all eternity, without beginning or end or how he foreknows us, how he brought the universe into being or how he listens to millions of prayers at once. Yet we believe all these things because he has told us in his Word, revealed himself in the person of his Son and has sent his Spirit into our hearts and one day our faith will turn to sight (Matt 5:8; 1 Cor 13:12; 1 John 3:2). In another sense, the Trinity is not such a hard concept because the fundamental principles are taught so clearly; there is only one Being who is the Creator God and his divine essence and identity is expressed distinctively as Father, Son and Holy Spirit, in perfect unity. Let us praise, worship and glorify our God; and may “the grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all.”
- Roger Nicole “The Meaning of the Trinity.” In One God in Trinity, ed. Peter Toon (Westchester Ill: Cornerstone, 1980) 4.
- Richard Bauckham. Jesus and the God of Israel (Grand Rapids: Eerdmans, 2008)
- John Calvin, Institutes of the Christian Religion 2 vols. Trans Ford Lewis Battles. (London: Westminster John Knox, 2006) Book I 13:18