Why was salvation going to the whole human race unthinkable to Peter?

Peter's mindset was steeped in Judaism, legalism, Temple worship, Christ's earthly ministry, and the Old Testament covenant promises, so he assumed God was still dealing with Israel under those promises. The sudden shift to God bringing salvation to the whole human race was therefore unthinkable to him.

Les teaches that Peter was steeped in Judaism, legalism, Temple worship, and the covenant promises coming out of the Old Testament, and he was also steeped further along in Christ's earthly ministry. All of a sudden Peter had to admit that God was not dealing any more with the nation of Israel on the covenant promises, but instead He was out to bring salvation to the whole human race. That was unthinkable to him. Peter had a hard time with it, and even he hedged a little, acknowledging there are things hard to be understood, which the unlearned and unstable twist to their own destruction.

What was Peter steeped in before he had to understand salvation going to the whole human race?

Peter was steeped in Judaism, legalism, Temple worship, Christ's earthly ministry, and the Old Testament covenant promises. He had to admit God was no longer dealing with Israel on covenant promises but was bringing salvation to the whole human race, which was unthinkable to him.