What did Peter have to admit about God's plan?
He had to admit that God was no longer dealing with the nation of Israel on the Old Testament covenant promises, but was instead bringing salvation to the whole human race. This was unthinkable to Peter because he was steeped in Judaism, legalism, Temple worship, and Israel's covenant promises.
Les explains that Peter was steeped in Judaism, legalism, Temple worship, Christ's earthly ministry, and the covenant promises coming out of the Old Testament. Suddenly Peter had to admit that God was not dealing any more with the nation of Israel on the covenant promises, but instead was out to bring salvation to the whole human race—unthinkable to him. So Peter had a hard time with it and even hedged a little, speaking of things hard to be understood. The unlearned and unstable twist these Scriptures, and their end result is destruction.