In this fourth title in the series of contemporary Catholic classics, William K. McDonough offers a popular explanation of the meaning of the Trinity, ?the fundamental doctrine of the Catholic faith, ? and its relation to the body of Christian revelation. He presents a synthesis of the Catholic faith: the triune life of God; the Person of the Father, the Son and the Holy Spirit; creation; the Incarnation; mariology; divine adoption; divine indwelling; prayer, sacraments and sacramentals; love; life with God here and hereafter. McDonough relies on the theology of Saints John and Paul, the teachings of the Fathers of the church, and the spiritual writings of Sister Elizabeth of the Trinity, a contemporary of Saint Th'r'se of Lisieux. He sees the mystery of the Trinity as a secret'the Secret of Secrets'beyond all our guessing'that, once revealed to us, becomes the pattern for true Christian living.