Augustine appeals to Pope Boniface to protect Christ’s flock by correcting Pelagian writings that endanger the less learned. He rejects the accusation that Catholic teaching is Manichaean: creation, marriage, procreation, free will, and the Law are good, but Adam’s sin has wounded human nature, leaving people unable to live righteously and immortally without grace. Concupiscence remains after baptism but is not culpable unless embraced, while baptism removes guilt and is necessary even for infants, whose contrasting destinies serve as evidence of original sin and gratuitous grace. Against Pelagian claims that human beings can initiate and complete goodness through their own powers, Augustine argues that God’s inward action precedes every good will, desire, and work, without abolishing genuine human cooperation. The Law exposes sin; the Spirit enables obedience. The saints of the Old Testament were saved through faith in Christ, whose sacrifice—not sinfulness—is meant by scriptural references to “sin,” and no believer achieves perfect sinlessness before the resurrection. Drawing on Cyprian and Ambrose as witnesses to the Church’s ancient faith, Augustine defends continual prayer, confession, and dependence on forgiveness. He presents Catholic doctrine as a middle path between Manichaean contempt for nature and Pelagian denial of original sin and Christ’s necessary grace, and maintains that bishops may condemn heresy locally without a universal synod.