On the Grace of Christ, and on Original Sin

In the same book the same St. Ambrose says again: “Now if Peter fell, who said, 'Though all men shall be offended, yet will I never be offended,' who else shall rightly presume concerning himself? David, indeed, because he had said, 'In my prosperity I said, I shall never be moved,' confesses how injurious his confidence had proved to himself: 'You turned away Your face,' he says, 'and I was troubled.'” Pelagius ought to listen to the teaching of so eminent a man, and should follow his faith, since he has commended his teaching and faith. Let him listen humbly; let him follow with fidelity; let him indulge no longer in obstinate presumption, lest he perish. Why does Pelagius choose to be sunk in that sea whence Peter was rescued by the Rock?