A historical and systematic introduction to what the medieval philospher and theologian Thomas Aquinas (1225-74) said about faith in the Trinity. Gilles Emery OP provides an explanation of the main questions in Thomas's treatise on the Trinity in his major work, the Summa Theologiae. His presentation clarifies the key ideas through which Thomas accounts for the nature of Trinitarian monotheism. Emery focuses on the personal relations of the Father, Son, and Holy Spirit, both in their eternal communion and in their creative and saving action. By highlighting the thought of one of the greatest defenders of the doctrine of the Trinity, he enables people to grasp the classical Christian understanding of God.