Many non-Christians, and likely many Christians as well, are baffled by the Christian concept of the Trinity. We wonder how God can be three as well as one, and even if he is, what that can possibly have to do with anything. But in fact, the Trinity is central both to the biblical depiction of God and to the outworking of human life, as Christians understand it.
At the simplest level, the doctrine of the Trinity is an attempt to do justice to the way Scripture describes the true God. The Bible does not simply affirm some kind of divinity, but instead describes God with great specificity. First, the Old Testament insists that there is only a single God (see especially Deut. 6:4-5 and Isaiah 43-45), and it describes the character of this one God in terms of perfections that Christians call attributes—God is perfectly holy, perfectly loving, perfectly powerful, able to gain knowledge from and work in all places at once, always existing, etc. So far, so good. At the same time, the Old Testament also makes mention of a “Word” or “Son” and a “Spirit” in connection with the one true God, the Lord (see Psalms 33:6, 110:1-5).
The New Testament speaks much more directly of this Word/Son and this Spirit. The “Word” or Son was in the beginning with God and was God, but is somehow also distinct from him (see John 1:1-3, Heb. 1:1-3). This Word/Son has become human, taking the name Jesus and living among us on earth (see John 1:14-18 and Gal. 4:4-7). This Spirit has descended to earth as well (without becoming human as the Son did) to dwell within people (see Acts 2). The New Testament speaks of God, his Son, and his Spirit together, as persons who constitute a single God, not three different gods (see especially Matt. 28:19-20, 2 Cor. 13:14).
Furthermore, the Son, Jesus, speaks of God as his Father, and he indicates that we too can be children of God and call him our Father (see Gal. 4:6). But there is a crucial difference: He is the only natural Son, the one who has always been God’s Son, even before he became human. We become sons and daughters by adoption. At heart, Christian faith is about the eternal relationship between God the Father, his Son, and his Spirit. The human race was created so that we could share in that relationship and live out the implications of that relationship in the world God created for us, but we lost that divine fellowship through what Christians call “the fall.” Christian redemption consists of the Son’s coming to earth as a man to live, die, and be raised for us, and the Spirit’s coming to dwell within us, so that we may once again be sons and daughters of God.
How the Father, Son, and Spirit can constitute a single God rather than three is indeed a complicated and mysterious theological and philosophical question. (You can begin to see the answer when you recognize that all three of them possess the same perfections, the same attributes.) But as difficult as this question is, the doctrine of the Trinity is not at heart abstract, philosophical, or even mysterious. God has sent his only Son to die for us and his Spirit to live in us, so that we may be the sisters and brothers of that Son, and thus the sons and daughters of his Father. The Christian faith is at heart about our relationship to Christ, his Father, and his Spirit. And that relationship is the same one that the Father, Son, and Spirit have shared from all eternity past (see John 17:21-26). The Trinity is thus the key to the relationship that lies at the very heart of the Christian faith.
Donald Fairbairn, Ph.D. is the Robert E. Cooley Professor of Early Christianity at Gordon-Conwell Theological Seminary.
For Professor Fairbairn's fuller treatment of this topic:
Donald Fairbairn, Life in the Trinity (IVP Academic)
Professor Fairbairn's further recommended reading on the topic:
Michael Reeves, Delighting in the Trinity (IVP)
Fred Sanders, The Deep Things of God (Crossway)