That the Cause of Evil is the Free Judgment of the Will
4 But I also, as yet, although I said and was firmly persuaded, that Thou our Lord, the true God, who made not only our souls but our bodies, and not our souls and bodies alone, but all creatures and all things, were uncontaminable and inconvertible, and in no part mutable: yet understood I not readily and clearly what was the cause of evil. And yet, whatever it was, I perceived that it must be so sought out as not to constrain me by it to believe that the immutable God was mutable, lest I myself should become the thing that I was seeking out. I sought, therefore, for it free from care, certain of the untruthfulness of what these asserted, whom I shunned with my whole heart; for I perceived that through seeking after the origin of evil, they were filled with malice, in that they liked better to think that Your Substance did suffer evil than that their own did commit it.
5. And I directed my attention to discern what I now heard, that free will was the cause of our doing evil, and Your righteous judgment of our suffering it. But I was unable clearly to discern it. So, then, trying to draw the eye of my mind from that pit, I was plunged again therein, and trying often, was as often plunged back again. But this raised me towards Your light, that I knew as well that I had a will as that I had life: when, therefore, I was willing or unwilling to do anything, I was most certain that it was none but myself that was willing and unwilling; and immediately I perceived that there was the cause of my sin. But what I did against my will I saw that I suffered rather than did, and that judged I not to be my fault, but my punishment; whereby, believing You to be most just, I quickly confessed myself to be not unjustly punished. But again I said: “Who made me? Was it not my God, who is not only good, but goodness itself? Whence came I then to will to do evil, and to be unwilling to do good, that there might be cause for my just punishment? Who was it that put this in me, and implanted in me the root of bitterness, seeing I was altogether made by my most sweet God? If the devil were the author, whence is that devil? And if he also, by his own perverse will, of a good angel became a devil, whence also was the evil will in him whereby he became a devil, seeing that the angel was made altogether good by that most Good Creator?” By these reflections was I again cast down and stifled; yet not plunged into that hell of error (where no man confesses unto You), to think that You allow evil, rather than that man does it.
Source: Confessions (New Advent)