The orthodox tactic regarding the problem of evil was the concept of privatio boni, which claims that evil was not part of God’s creation at first. Evil comes about later, from the the voluntary actions of free-willed beings. Humans are mutable (changeable) because they are made out of nothing – they are contingent creatures that could always be different from the way they are and therefore they have the real possibility of becoming evil. A free-willed creature must have the possibility of turning away from God (which is what freewill means).
Plato argued that evil was merely the privation of good, that it had no ontological status of its own. While he of course acknowledged the existence of moral evils in the form of wars, lies, …he conveniently explained them away as a mere lack of peace or lack of truth. The
Christian theologians such as Augustine and Thomas Aquinas were quick to accept Plato’s position regarding evil, and lay spurious claim to his beliefs as if they were inherent in Christian theology.
They did not recognize the incompatibility of the privatio boni with the concepts of active evil, and, again, the omnipotence of God. For if evil has no being, how can it have a principle such as that incarnated in the Devil?
Worse still, the dismissal of evil as an “accidental lack of perfection” flaw in the face of both reason and intuition as there is no room for such failure from an omniscient God.
For centuries afterward, Catholic scholars would debate the subtleties of the Devil’s origin, fall and subsequent status in the creation, without ever facing the basic contradiction of divine benevolence with the presence of evil.
The solution had, in fact, been provided in the first two centuries after Christ, but was declared heretical and suppressed by the Church. This was the mythological system of Christian Gnosticism.