But if you depreciate the littleness of man, not on account of his body, but of his soul, regarding it as inferior to that of other rational beings, and especially of those who are virtuous; and inferior, because evil dwells in it—why should those among Christians who are wicked, and those among the Jews who lead sinful lives, be termed a collection of bats, or ants, or worms, or frogs, rather than those individuals among other nations who are guilty of wickedness?— seeing, in this respect, any individual whatever, especially if carried away by the tide of evil, is, in comparison with the rest of mankind, a bat, and worm, and frog, and ant.
And although a man may be an orator like Demosthenes, yet, if stained with wickedness like his, and guilty of deeds proceeding, like his, from a wicked nature; or an Antiphon, who was also considered to be indeed an orator, yet who annihilated the doctrine of providence in his writings, which were entitled Concerning Truth, like that discourse of Celsus,— such individuals are notwithstanding worms, rolling in a corner of the dung-heap of stupidity and ignorance. Indeed, whatever be the nature of the rational faculty, it could not reasonably be compared to a worm, because it possesses capabilities of virtue. For these adumbrations towards virtue do not allow of those who possess the power of acquiring it, and who are incapable of wholly losing its seeds, to be likened to a worm.
It appears, therefore, that neither can men in general be deemed worms in comparison with God. For reason, having its beginning in the reason of God, cannot allow of the rational animal being considered wholly alien from Deity. Nor can those among Christians and Jews who are wicked, and who, in truth, are neither Christians nor Jews, be compared, more than other wicked men, to worms rolling in a corner of a dunghill. And if the nature of reason will not permit of such comparisons, it is manifest that we must not calumniate human nature, which has been formed for virtue, even if it should sin through ignorance, nor liken it to animals of the kind described.
Source: Against Celsus (New Advent)