From Bob Hill of the now discontinued Biblical Answers:
Augustine also explained the reasoning which allowed him to be converted to the Catholic faith:
For first of all the things began to appear unto me as possible to be defended: and the Catholic faith, in defense of which I thought nothing could be answered to the Manichees’ arguments, I now concluded with myself, might well be maintained without absurdity: especially after I had heard one or two hard places of the Old Testament resolved now and then; which when I understood literally, I was slain. Many places therefore of those books having been spiritually expounded.
What could Augustine not accept literally? One of them was the mutability of God, that God would change his will or purpose from one time to the next in order to adjust to a changeable mankind. In Confessions, Augustine explains which literal interpretations were unacceptable. Here is one of his statements:
And because God commanded them one thing then, and these another thing now for certain temporal respects; and yet those of both ages were servants to the same righteousness, whereas they may observe in one man, and in one day, and in one house, different things to be fit for different members, and one thing to be lawful now, which in an hour hence is not so; and something to be permitted or commanded in one corner, which is forbidden or punished in another. Is Justice thereupon various or mutable.
The Manichaeans believed God could not be mutable and retain his perfection. Augustine accepted this philosophy as true and attempted to prove this doctrine with Scripture.
In another writing, On the Morals of the Catholic Church, Augustine explained the doctrines of the Old Testament that were so absurd. In explaining his dispute with the Manichaeans we observe his agreement with them against the literal interpretation of the Old Testament.
We do not worship a God who repents, or is envious, or needy, or cruel, or who takes pleasure in the blood men or beasts, or is pleased with guilt or crime, or whose possession of the earth is limited to a little corner of it. These and such like are the silly notions . . . the fancies of old women or of children . . . and in those by whom these passages are literally understood. . . . And should any one suppose that anything in God’s substance or nature can suffer change or conversion, he will be held guilty of wild profanity.
Augustine agreed with the Manichaeans that a mutable God was totally unacceptable. In this conflict between the Platonic doctrine of immutability and the literal interpretation of Scriptures what had to change? Augustine’s answer was that the literal interpretation of Scripture had to change. For Augustine the plain narratives of Scripture had to be reinterpreted by spiritual or allegorical methods. The Manichaeans believed the Old Testament revealed a God who was mutable or could repent. Since the Platonists believed that God was immutable this idea of God repenting was a source of ridicule for the Catholic Church. Augustine was so embarrassed by these arguments that he chose to reinterpret Scripture rather than refute the Platonic philosophy.