Augustine argued that grace not only facilitates our efforts to obey God, but because of our fallen nature, grace is necessary. Before the fall, the requirement for moral perfection was already present. The fall did not change the requirement, but it did change us. What was once a moral possibility became, without grace, a moral impossibility. Augustine’s view is rooted in his doctrine of original sin. As the debate escalated, Pelagius aimed his guns at this doctrine.
Sproul, R.C.; Grace Unknown: The Heart of Reformed Theology; Baker Books; Grand Rapids, MI; copyright 1997; p. 122