Many studies of spiritual development exist under the heading of "Christian Perfection." However, John Gavin revisits such topics as asceticism, prayer, sacraments, virtues, and spiritual combat through scriptural and patristic texts that present the Christian life as one of growth from spiritual infancy to a particular fulfilment or end (telos): divinized humanity as formed and revealed in Jesus Christ. Thus, though Christian maturity does incorporate such things as physical and cognitive development, its true distinction lies in its gifted, supernatural end that does not exclude human freedom.

Part One establishes the pillars of Christian maturity – form and finality; virtue and character; vocation and mission – and explores the opposition to maturation in the form of demonic infantilism. Part Two examines the means of maturity given to us in the life of the Church: the Scriptures, the Mysteries (Sacraments), and asceticism. Finally, Part Three reviews four figures of Christian adulthood: the Witness, the Teacher, the Servant, and the Fool. A concluding chapter applies the insights from the previous chapters to our modern world to see in what ways our times need to "grow up."

Growing into God includes a variety of early Christian voices: Irenaeus of Lyons, Origen of Alexandria, Gregory of Nazianzus, Gregory of Nyssa, Basil of Caesarea, Augustine of Hippo, Ambrose of Milan, John Cassian, Dionysius the Areopagite, Mark the Monk, John Moschos, Maximus the Confessor, Symeon the Holy Fool, and others. Their collective insights, all the fruits of great learning and the contemplation of God's Word, describe a wondrous figure: the mature saint transformed by union with the Father, Son, and Spirit.