What we believe about the Trinity sums up much of what we believe about God.
At Pentecost we celebrated the gift of the Holy Spirit. At Easter we gave thanks for the victory of our Lord Jesus Christ, while at Christmas we focussed on God Emmanuel (God with us). The Trinity expresses our belief in God the Creator, Redeemer, Sustainer. You won’t find this spelled out in the New Testament in so many words, but it is there. Just think of what we say in the grace: “The grace of our Lord Jesus Christ, the love of God and the fellowship of the Holy Spirit” (2 Corinthians 13.13). Think too of how God, Jesus and the Spirit are all mentioned in the great commission at the end of Matthew’s gospel (28.19). Belief in the Trinity helps us make sense of such things as the baptism and temptations of our Lord, his pleading with God in the garden of Gethsemane and the cry of agony and despair from the cross: ‘My God, my God, why have you forsaken me’ (Matt.27.47), not to speak of God’s raising Jesus from the dead. In fact, the doctrine of the Trinity revolutionizes our understanding of God and our attitude to him. It explains how God is love by pointing us to Jesus.
How did the early Christians come to think of God in three ways? They began of course with God. They took over the Jewish belief in one God. But they had to account for Jesus Christ. Perhaps this was not as difficult as we imagine. They had the prophecy of the suffering servant in Isaiah 53. Then before they knew what was happening there was more to reckon with, for they found themselves enveloped in the experience of the Holy Spirit (Acts 2). Once again they had the scriptures to help them. Prophets had foretold that in the coming age God would pour out his spirit in unprecedented fashion (Ezek.23.1-14; Joel 2.28-32).
Thus the doctrine of the Trinity is based on both human experience and the testimony of scripture. It enshrines what we believe about God who is beyond our imagination and understanding but who has made himself known in Jesus Christ and who continues to reveal himself through the Holy Spirit. Hence in worship we never tire of saying ‘Glory be to God the Father and to the Son and the Holy Spirit.’
God in three persons. Our belief in the Trinity helps us understand puzzling events in the story of Jesus’ life, like his baptism and cry from the cross. The Trinity as a community of persons speaks to us in an age like ours when individualism and loneliness are so prevalent. Our well-being as individuals depends upon the well-being of others, and their well-being depends on us. An individual offering friendship and help can make all the difference, not only to the receiver but also to the giver. But since exclusiveness can be just as much of a problem for two people as for one person, a third person is essential for encouraging inclusiveness. The Trinity keeps reminding us of the outgoing and inclusive nature of Christian love.