The central mystery of Christian faith and life is the revelation that God is Father, Son, and Holy Spirit. Seeking to understand the Trinitarian faith, definitively expressed in the Ecumenical Councils, requires a consideration both of what is common to the Three Persons, as well as of what is proper to each. The harmony of faith and reason will be a constant theme. Saint Thomas Aquinas’s treatment of the mystery of God in the Summa Theologiae will serve as the key text for this course. We will pay special attention to the spiritual analogy for the processions in God, while also exploring the complementary path to unfolding the imago Trinitatis in terms of the communion of persons. We will end with a consideration of several contemporary authors.