This dissertation explores the intellectual context of the Pelagian controversy of the early fifth century. The Pelagian debate tried to establish the relationship between the sovereignty of God and the agency of humans; it was a Christian problem, but the participants in the debate were informed by contemporary Neoplatonism, an aspect of Pelagianism that scholars have not sufficiently addressed. Both Pelagius and his disciple, Caelestius, appropriated Neoplatonic forms and ethics in order to express their affirmation of mankind's ability to choose and merit salvation. Though Pelagius' teaching has traditionally been labeled a Western heresy, it actually received a mix of friendly and hostile reactions among the churches in Jerusalem, Ephesus, and Constantinople. Pelagianism was not therefore simply a Western phenomenon, as some scholars have supposed. Rather, the careers of Caelestius and another famous Pelagian, Julian of Eclanum, exposed a commonality of interest throughout different spheres of ecclesiastical Mediterranean authority. There were also various attempts to find a middle ground between the Augustinian and Pelagian positions. One of those attempts is evident in the mosaics of S. Maria Maggiore in Rome. These mosaics were commissioned by Pope Sixtus III, whose papacy indicates that there was a significant level of discomfort among the Roman clergy with the extremes of both Pelagian and Augustinian theology.