After this Celsus continues: “It is not easy, indeed, for one who is not a philosopher to ascertain the origin of evils, though it is sufficient for the multitude to say that they do not proceed from God, but cleave to matter, and have their abode among mortal things; while the course of mortal things being the same from beginning to end, the same things must always, agreeably to the appointed cycles, recur in the past, present, and future.” Celsus here observes that it is not easy for one who is not a philosopher to ascertain the origin of evils, as if it were an easy matter for a philosopher to gain this knowledge, while for one who is not a philosopher it was difficult, though still possible, for such an one, although with great labour, to attain it.
Now, to this we say, that the origin of evils is a subject which is not easy even for a philosopher to master, and that perhaps it is impossible even for such to attain a clear understanding of it, unless it be revealed to them by divine inspiration, both what evils are, and how they originated, and how they shall be made to disappear. But although ignorance of God is an evil, and one of the greatest of these is not to know how God is to be served and worshipped, yet, as even Celsus would admit, there are undoubtedly some philosophers who have been ignorant of this, as is evident from the views of the different philosophical sects; whereas, according to our judgment, no one is capable of ascertaining the origin of evils who does not know that it is wicked to suppose that piety is preserved uninjured amid the laws that are established in different states, in conformity with the generally prevailing ideas of government. No one, moreover, who has not heard what is related of him who is called “devil,” and of his “angels,” and what he was before he became a devil, and how he became such, and what was the cause of the simultaneous apostasy of those who are termed his angels, will be able to ascertain the origin of evils. But he who would attain to this knowledge must learn more accurately the nature of demons, and know that they are not the work of God so far as respects their demoniacal nature, but only in so far as they are possessed of reason; and also what their origin was, so that they became beings of such a nature, that while converted into demons, the powers of their mind remain. And if there be any topic of human investigation which is difficult for our nature to grasp, certainly the origin of evils may be considered to be such.
Source: Against Celsus (New Advent)