We do not have many definitions of Christianity from late antiquity, but among the few extant is the brief statement of Gregory of Nyssa (335-395 CE) that it is "mimesis of the divine nature." The sentence is both a historical gem and theologically puzzling. Gregory was the first Christian to make the infinity of God central to his theological program, but how could he intend for humans to imitate the infinite? If the aim of the Christian life...