24 The prophecy of the seventy weeks shows that the prophet looked to God alone for the establishment of righteousness.
So then, until that end shall come, and this corruptible and mortal shall put on incorruption and immortality, we must be liable to sin; not, as you falsely say, owing to the fault of our nature and creation, but through the frailty and fickleness of human will, which varies from moment to moment; because God alone changes not. You ask in what respects Abel, Enoch, Joshua the son of Nun, or Elisha, and the rest of the saints have sinned. There is no need to look for a knot in a bulrush; I freely confess I do not know; and I only wish that, when sins are manifest, I might still be silent. “I know nothing against myself,” says St. Paul, “yet am I not hereby justified.” “Man looks on the outward appearance, but the Lord looks on the heart.” Before Him no man is justified. And so Paul says confidently, “All have sinned, and come short of the glory of God”; and “God has shut up all under sin that He may have mercy upon all”; and similarly in other passages which we have repeated again and again.
Source: Against the Pelagians (New Advent)