In The City of God, Augustine discredits Roman Paganism on its own terms by arguing at length that the gods are evil and petty, as shown through the myths, etc. On face value, he's assuming their existence, but only for the purpose of showing their absurdity.
In other places, like the one I quoted above, he unequivocally denies that they are gods. They are demons posing as gods, and the myths are false.