From his great work City of God, II, i:
Pretty spot on. Here it is in its whole context from another translation:
If the feeble mind of man did not presume to resist the clear evidence of truth, but yielded its infirmity to wholesome doctrines, as to a health-giving medicine, until it obtained from God, by its faith and piety, the grace needed to heal it, they who have just ideas, and express them in suitable language, would need to use no long discourse to refute the errors of empty conjecture. But this mental infirmity is now more prevalent and hurtful than ever, to such an extent that even after the truth has been as fully demonstrated as man can prove it to man, they hold for the very truth their own unreasonable fancies, either on account of their great blindness, which prevents them from seeing what is plainly set before them, or on account of their opinionative obstinacy, which prevents them from acknowledging the force of what they do see. There therefore frequently arises a necessity of speaking more fully on those points which are already clear, that we may, as it were, present them not to the eye, but even to the touch, so that they may be felt even by those who close their eyes against them. And yet to what end shall we ever bring our discussions, or what bounds can be set to our discourse, if we proceed on the principle that we must always reply to those who reply to us? For those who are either unable to understand our arguments, or are so hardened by the habit of contradiction, that though they understand they cannot yield to them, reply to us, and, as it is written, "speak hard things," and are incorrigibly vain. Now, if we were to propose to confute their objections as often as they with brazen face chose to disregard our arguments, and as often as they could by any means contradict our statements, you see how endless, and fruitless, and painful a task we should be undertaking. And therefore I do not wish my writings to be judged even by you, my son Marcellinus, nor by any of those others at whose service this work of mine is freely and in all Christian charity put, if at least you intend always to require a reply to every exception which you hear taken to what you read in it; for so you would become like those silly women of whom the apostle says that they are "always learning, and never able to come to the knowledge of the truth."