Augustine of Hippo, On the Spirit and the Letter Chapter 20: The Law Without Grace

Now why need I speak of what follows? For why it was that by this their impiety those men—I mean those who could have known the Creator through the creature—fell (since “God resisteth the proud”) and whither they plunged, is better shown in the sequel of this epistle than we can here mention. For in this letter of mine we have not undertaken to expound this epistle, but only mainly on its authority, to demonstrate, so far as we are able, that we are assisted by divine aid towards the achievement of righteousness,—not merely because God has given us a law full of good and holy precepts, but because our very will without which we cannot do any good thing, is assisted and elevated by the importation of the Spirit of grace, without which help mere teaching is “the letter that killeth,” forasmuch as it rather holds them guilty of transgression, than justifies the ungodly. Now just as those who come to know the Creator through the creature received no benefit towards salvation, from their knowledge,—because “though they knew God, they glorified Him not as God, nor gave Him thanks, although professing themselves to be wise;”—so also they who know from the law how man ought to live, are not made righteous by their knowledge, because, “going about to establish their own righteousness, they have not submitted themselves unto the righteousness of God.”