50 You say that there are good men in the human race; and perhaps, if we compare them with the very wicked, we may be led to believe that there are. Who are they, pray? Tell us. The philosophers, I suppose, who assert that they alone are most wise, and who have been uplifted with pride from the meaning attached to this name, — those, forsooth, who are striving with their passions every day, and struggling to drive out, to expel deeply-rooted passions from their minds by the persistent opposition of their better qualities; who, that it may be impossible for them to be led into wickedness at the suggestion of some opportunity, shun riches and inheritances, that they may remove from themselves occasions of stumbling; but in doing this, and being solicitous about it, they show very clearly that their souls are, through their weakness, ready and prone to fall into vice.
In our opinion, however, that which is good naturally, does not require to be either corrected or reproved; nay more, it should not know what evil is, if the nature of each kind would abide in its own integrity, for neither can two contraries be implanted in each other, nor can equality be contained in inequality, nor sweetness in bitterness. He, then, who struggles to amend the inborn depravity of his inclinations, shows most clearly that he is imperfect, blameable, although he may strive with all zeal and steadfastness.
Source: Against the Heathen (New Advent)