The mystery of the trinitarian God is the central belief of Christianity. This book sets out to show how the doctrine of the Trinity is rooted in God's revelation in the Bible, as well as in the experience of Christians and has found expression in a long line of Christian writers and thinkers. Edmund Hill begins with the statements of the doctrine in the early creeds and goes on to investigate the New Testament sources and the early Fathers. But is in St Augustine and especially in the great treatise De Trinitate that he finds the fullest expression of the mystery of the Trinity. The central part of the book is therefore a lucid and compelling commentary on st Augustine's teaching, thought one that also takes account of later thinking, especially that of Thomas Aquinas. This is followed by a consideration of the trinitarian image of God as a program for Christian living, a critique of inadequate catechism formulae and a new catechism of the doctrine of the Trinity.