Paul and Peter are undoubtedly proved to have been men of a spiritual nature. When, therefore, Paul is found to have acted contrary to religion, in having persecuted the Church of God, and Peter to have committed so grave a sin as, when questioned by the maid-servant, to have asserted with an oath that he did not know who Christ was, how is it possible that these— who, according to those persons of whom we speak, were spiritual beings— should fall into sins of such a nature, especially as they are frequently in the habit of saying that a good tree cannot bring forth evil fruits? (Book I, Chapter 8, Part 2)

I have never encountered a good tree that has not produced some bad fruit.

The bad fruit may derive from lack of rain, or worms, or hail, or frost, or excessive heat, or being overgrown, or any number of causes.

The tree is good, but at least some of its fruit is bad.