What was the hard thing Peter could not understand about Paul's message?
The hard thing was that God would save a multitude of Gentiles without the Jews. Paul’s epistles spoke of these salvation truths, yet some things were hard to be understood. Even at that late date Peter could not comprehend this inclusion of Gentiles apart from Israel.
Les explains that Peter referred to Paul’s epistles, Romans through Hebrews, and acknowledged that some things in them are hard to be understood. Peter was in the same boat as people who struggle with these teachings because they are not used to them. The specific difficulty was that God would save a multitude of Gentiles without the Jews—that was Paul’s message. Even at that late date, Peter just could not comprehend it. Les further notes that the unlearned and unstable twist Paul’s epistles as they do the other Scriptures, and cults become masters at twisting Scripture, resulting in their own destruction.