“How great is the frailty of man, ever prone to evil! Today you confess your sins; tomorrow you again commit the very sins you have confessed! Now you resolve to guard against them, and within the hour you act as though you had never made any resolution! Remembering, then, our weakness and instability, it is proper to humble ourselves and never to have a high opinion of ourselves.”
Thomas a Kempis, the Imitation of Christ, from the excerpt in A Reformation Reader: Primary Texts with Introductions
A Kempis was correct, though he should have added that we also ought to constantly look to Christ, not only as our example, but also as the one whose perfect righteousness is ours.