Augustine answers a question put to him by his friend Marcellinus: how can he hold that a sinless life is possible by God's help, yet maintain that no one has ever lived without sin? The reply turns on the contrast Paul draws in 2 Corinthians 3, where the letter kills but the Spirit gives life. Written in 412 as the Pelagian controversy opened, it argues that the law exposes sin but cannot cure it, and that righteousness comes only through the grace of the Holy Spirit poured into the heart.

A point by point reply to a lost book by Pelagius that argued human nature retains the full power to avoid sin. Augustine concedes the goodness of nature as created, then shows it wounded by the fall and unable to heal itself without the grace given in Christ. Composed around 415, it became one of the central texts of the Western doctrine of original sin and the necessity of grace, and it is the work in which Augustine first quotes Pelagius at length to refute him from his own words.

Written for the monks of Hadrumetum, who had fallen into dispute over whether grace destroys human freedom. Augustine sets out to show that Scripture affirms both the reality of free choice and the absolute necessity of grace, and that the two are not rivals. He gathers the commands and the promises of Scripture side by side, arguing that God crowns his own gifts when he rewards human merits, since even a good will is itself the work of grace.

A companion piece to On Grace and Free Will, also sent to Hadrumetum, where some had concluded that if grace governs all things then rebuke of the erring is pointless. Augustine answers that correction is itself one of the means God uses to call his elect, and he draws here his mature distinction between the grace given to Adam, which made it possible not to sin, and the greater grace of glory, which will make it impossible to sin. The treatise gives one of his fullest accounts of the gift of perseverance.

Addressed to Prosper and Hilary, who had reported that monks in southern Gaul accepted the need for grace but held that the first movement of faith comes from the unaided human will. Augustine replies that faith itself is a gift, and that God's choice of those who will believe precedes and produces their believing. Written near the end of his life, it traces predestination to the free mercy of God and grounds the believer's assurance in election rather than in human initiative.

The sequel to On the Predestination of the Saints and the last sustained work of the Pelagian controversy, written about a year before Augustine's death. It answers the objection that even if faith is given by grace, perseverance to the end must depend on the believer. Augustine argues that final perseverance is no less a gift than faith, asked for daily in the Lord's Prayer, and that those who fall away show they were never numbered among the predestined.