Celsus has made a statement regarding evils of the following nature, viz., that “although a thing may seem to you to be evil, it is by no means certain that it is so; for you do not know what is of advantage to yourself, or to another, or to the whole world.” Now this assertion is made with a certain degree of caution; and it hints that the nature of evil is not wholly wicked, because that which may be considered so in individual cases, may contain something which is of advantage to the whole community.
However, lest any one should mistake my words, and find a pretence of wrongdoing, as if his wickedness were profitable to the world, or at least might be so, we have to say, that although God, who preserves the free-will of each individual, may make use of the evil of the wicked for the administration of the world, so disposing them as to conduce to the benefit of the whole; yet, notwithstanding, such an individual is deserving of censure, and as such has been appointed for a use, which is a subject of loathing to each separate individual, although of advantage to the whole community. It is as if one were to say that in the case of a city, a man who had committed certain crimes, and on account of these had been condemned to serve in public works that were useful to the community, did something that was of advantage to the entire city, while he himself was engaged in an abominable task, in which no one possessed of moderate understanding would wish to be engaged.
Paul also, the apostle of Jesus, teaches us that even the very wicked will contribute to the good of the whole, while in themselves they will be among the vile, but that the most virtuous men, too, will be of the greatest advantage to the world, and will therefore on that account occupy the noblest position. His words are: “But in a great house there are not only vessels of gold and silver, but also of wood and of earth; and some to honour, and some to dishonour. If a man therefore purge himself, he shall be a vessel unto honour, sanctified and meet for the Master's use, prepared unto every good work.” These remarks I have thought it necessary to make in reply to the assertion, that “although a thing may seem to you to be evil, it is by no means certain that it is so, for you do not know what is of advantage either to yourself or to another,” in order that no one may take occasion from what has been said on the subject to commit sin, on the pretext that he will thus be useful to the world.
Source: Against Celsus (New Advent)