The Enchiridion by St. Augustine
Chapter 81.--There are Two Causes of Sin, Ignorance and Weakness; And We Need Divine Help to Overcome Both.
I shall now say this, which I have often said before in other places of my works. There are two causes that lead to sin: either we do not yet know our duty, or we do not perform the duty that we know. The former is the sin of ignorance, the latter of weakness. Now against these it is our duty to struggle; but we shall certainly be beaten in the fight, unless we are helped by God, not only to see our duty, but also, when we clearly see it, to make the love of righteousness stronger in us than the love of earthly things, the eager longing after which, or the fear of losing which, leads us with our eyes open into known sin. In the latter case we are not only sinners, for we are so even when we err through ignorance, but we are also transgressors of the law; for we leave undone what we know we ought to do, and we do what we know we ought not to do. Wherefore not only ought we to pray for pardon when we have sinned, saying, |Forgive us our debts, as we forgive our debtors;| but we ought to pray for guidance, that we may be kept from sinning, saying, |and lead us not into temptation.| And we are to pray to Him of whom the Psalmist says, |The Lord is my light and my salvation:| my light, for He removes my ignorance; my salvation, for He takes away my infirmity.