Augustin's Opposition to Baptism

Theological conviction and personal experience taught him that the tendency toward sin is an unavoidable part of human life, not something that can be washed away in a single ritual. In Augustin's view, the Pelagians, like the Donatists, were undermining the whole point of the Christian faith. If as they claimed, the human will is sovereign and capable of resisting sin all on its own, why does God need to send his son and allow him to be sacrificed?