Ovid’s Metamorphoses, the encyclopedia of pagan mythology for later centuries, was also pressed into the service of Christian theology. This chapter shows how three Christian poets—Prudentius, Dante, and John Milton—reworked metamorphosis into a snake, each in his own specific way. The central text is Metamorphoses 4.569–603, where Cadmus and his wife Harmonia are transformed into snakes in fulfilment of a supernatural prophecy, the ultimate consequence of Cadmus’ slaying of the serpent of Mars on the site of Thebes. While the metamorphosed Cadmus falls physically to the ground, in the Christian authors serpentine metamorphosis signifies a theological and spiritual fall. In the late fourth century, Prudentius projects the transformation on to Satan, while Dante in the early fourteenth century exploits Ovidian metamorphosis into snakes for the punishment of sinners in Inferno. Milton, a reader of both Prudentius and Dante, applies the metamorphosis to Satan, but in a very different way from Prudentius. Each of these Christian poets explores the motif of serpentine metamorphosis in their own language, and within their own culture. The demonic repetition of falls into serpentine metamorphosis also figures the repetitive migration of the Ovidian motif through the Christian centuries.