Why was it hard or unthinkable for Peter to accept that God was now bringing salvation to the whole human race?

Les explains that from Genesis 3 to the end of Scripture, the Book is about God bringing salvation to a fallen human race, with Christ on almost every page. Peter, however, was steeped in Judaism, legalism, Temple worship, Christ's earthly ministry, and the Old Testament covenant promises. Therefore, when he had to admit God was no longer dealing with Israel on those covenant promises but was bringing salvation to the whole human race, it was unthinkable to him. He had a hard time with it and hedged, since even for a man like Peter these things were hard to be understood; the unlearned and unstable twist them to destruction.