One of the most difficult problems in both philosophy and religion is that of how to deal with evil and suffering in the world while still asserting that God exists, is powerful enough to be creator, and is good. Both St. Augustine of Hippo, in Book VII of his landmark work The Confessions, and St. Thomas Aquinas, throughout his writings, but particularly in his work On Evil, seek to address this important topic. Both works provide answers to the difficulty from both reason and revelation, but it is St. Augustine's theory of evil as the privation of the good that is one of the most persuasive offered from the medieval philosophers.
St. Augustine begins his discussion on the topic of evil with an argument against the notion prevalent among the pagan and Manichaen opponents of the Christians whom he was battling. According to different schools within those two traditions, evil can be accounted as an equal and opposing force to good; it is something to be battled, but never defeated. Because of this, evil is a necessary contrary to good in the same way that hot and cold, dry and wet, and other such contraries were understood in the Aristotelian and Platonic philosophies of the time. St. Augustine rejects this understanding and advances the idea that evil is not necessary. Instead, he argues, evil is not actually a substance or thing, but that it is an absence of good. St. Augustine posits that being, having been created by God, is inherently good, even despite the corrupting influence of the fall and the subsequent introduction into creation of concupiscence. Rather than being the opposite of good, evil is, according to St. Augustine, an absence of good—the non-good or privation of the good.
Since evil is not actually a thing per se, and is rather the absence of the good, St. Augustine argues that God did not cause evil. When God created the universe, everything was good, because it had being given to it by God in the creatio ex nihilo. When Satan and his angels rebelled, and then when the progenitors of the human race similarly chose to sin, evil entered into the world in the sense that the free choices of these rational creatures, who share in God's essence for their being because of their radical contingency in God, caused what could be thought of as a “bubble” of non-being to enter into the universe of being. It need not have been so. Instead of exercising the free will given to them by the God in whom they have their being, both the angels and men could have chosen to remained in God's will, cooperating with the good.
On the other hand, contra St. Augustine, it could be argued that while God was not directly the cause of evil, since God's creation of the universe out of nothing resulted in being, which St. Augustine defines as good, God's hand in the creation of rational creatures who could choose the not-good, or evil, makes him at least an indirect cause (though not necessarily responsible). St. Augustine would reject this premise, because he would be loathe to associate God with even the possibility of his being responsible for evil, since in St. Augustine's conception, God is all-good. But it is difficult to see an acceptable interruption in the chain of causality linking God's necessary instrumentality in the causing (creation) of the universe and the resultant evil which arose due to the God-given free will of the rational creatures that populate the universe.
If, as St. Augustine asserts, God is not the cause or responsible party for evil, but there exists evil in the world, God would seem to be impotent and not all-powerful, as St. Augustine would posit, evil, or non-being, does exist (which G.K. Chesterton called the one empirically provable aspect of Christian doctrine). St. Augustine would argue that God does allow evil to exist (or rather, since evil is non-being, God allows pockets or bubbles of non-being to avoid being filled with the fullness of being that is goodness). As already stated above, God has to allow the possibility of evil in order to grant his creatures true free will: for if a creature could not choose the non-being (which is the non-good, or evil), then they would not have free will. Thus, the problem of evil can be addressed this way: God created all being, and all being is good. God endowed his rational creatures with essence, existence, and free will. In order for his creatures to exercise free will, God must allow them to choose to reject the good, which is the non-being of evil. This is the source of evil in our universe. God could stop it, but, in his sovereignty, he chooses to allow evil in the short turn to honor the choices of his creatures. To mitigate this, he promises that even short-term evil will ultimately bring about good, so that in the end, Jesus Christ—God's full, self-revelation—will be all in all.