What is the role of divine grace in sanctifying the will and opposing the role of the devil in corrupting it?

St. Jerome says, “Let us ask a question of the heretics, who claim that there are two mutually opposed natures [of souls]. If in accordance with their understanding, a good tree can never produce evil fruit [even if it deviates], how is it that Moses, who was a good tree, sinned at the water of contradiction (cf. Deut. 32:51)? How is it that … Peter too, at the Lord’s Passion, denied Him, saying: ‘I do not know the man’ (Matt. 26:72). Or with what consistency did Jethro, the father-in-law of Moses, an evil tree who assuredly did not even believe in the God of Israel, give good counsel to Moses (Ex. 18:19)?”¹⁰⁶⁶