Why did Peter say Jesus' return would be delayed until the tribulation even if Israel repented?
Because Daniel 9 foretold the Antichrist would defile the temple in the middle of the final week, and Jesus referenced this in Matthew 24. These events had to happen, so even if Israel repented, the return had to wait until the tribulation, called the restitution of all things.
Les teaches that Peter’s statement in verse 21 rested on covenant ground. It was all determined in Daniel 9, about verse 25: in the middle of the final week the Antichrist would come into the temple and defile it, which the Lord Jesus also referenced in Matthew 24. Peter knew all these things had to happen, so even though the Lord was ready to return if Israel would repent, it still had to wait until that period of restitution of all things, which is the tribulation. Les notes Peter is still speaking to the great crowd of Jews and then goes back to Moses as Israel’s deliverer and redeemer, who brought them out of slavery and captivity unto Abraham’s God. Moses was the deliverer in type. Peter remains on covenant ground. The follow-up in verse 23 is that except a man be born again, he cannot enter the Kingdom of Heaven. If Israel had repented, no unbelieving Jews would go in; every Jew was admonished to become a believer, and no unbeliever will ever enter the Kingdom of Heaven.