Augustine on Sonship Theology

God is the source of the new heart and Christian obedience
What does the putrid flesh of humanity have left to puff itself up with, and to refuse to glory in the Lord? Whatever it claims it has done to achieve virtue by its own effort, so that God must then reward it — against all such claims it shall be answered, it shall be exclaimed, it shall be contradicted, ‘I do it; but for My own holy name’s sake; I do not do it for your sakes, says the Lord God’ (Ezek.36:22). Nothing so overthrows the Pelagians when they say that the grace of God is given according to our virtues. (In fact, Pelagius himself condemned this view, although he did not embrace the correct one — he was just afraid of the Eastern judges.) Nothing so overthrows the arrogance of people who say, ‘We do it, that we may by our virtues establish a basis for God to work.’ It is not Pelagius that answers you, but the Lord Himself: ‘I do it, and not for your sakes, but for My own holy name’s sake.’ For what good can you do out of a heart that is not good? But in order that you may have a good heart, He says, ‘I will give you a new heart, and I will put a new spirit within you’ (Ezek.36:26).

Can you say, ‘We will first walk in His righteousness, and will observe His judgments, and will act in a worthy way, so that He will give His grace to us’? But what good would you evil people d? And how would you do those good things, unless you were yourselves good? But Who causes people to be good? Only He Who said, ‘And I will visit them to make them good,’ and, ‘I will put my Spirit within you, and will cause you to walk in my righteousness, and to observe my judgments, and do them’ (Ezek.36:27). Are you asleep? Can’t you hear Him saying, ‘I will cause you to walk, I will make you to observe,’ lastly, ‘I will make you to do’? Really, are you still puffing yourselves up? We walk, true enough, and we observe, and we do; but it is God Who He makes us to walk, to observe, to do. This is the grace of God making us good; this is His mercy going before us.

Against Two Letters of the Pelagians, 4:15 (Thanks to Dr. Needham)