What was the hard-to-understand part of Paul's message for Peter?
Peter struggled to comprehend that God would save a multitude of Gentiles without the Jews, which was Paul's message. Paul's epistles contained some things hard to be understood, even at that late date, because Gentiles being saved without Jews was difficult for Peter to accept.
Les explains that Peter acknowledged Paul wrote according to the wisdom given him, and in all his epistles Paul spoke of salvation. Yet Peter said some things in Paul's epistles are hard to be understood. Peter was in the same boat as those who have problems with teaching they are not used to: he could not comprehend that God would save a multitude of Gentiles without the Jews. That was Paul's message, and even at that late date Peter just could not comprehend it. Les also notes that the unlearned and unstable twist these things, as they do other Scriptures, and that such twisting leads to their own destruction.