What would have happened if all Israel repented after Peter's Acts 2 message?
If every Israelite had repented of their national sin and recognized that the one they killed was indeed their Messiah and King, God would have sent Christ at that time to be their King and set up the kingdom. Peter's Acts 2 call was to repent and be baptized, every one of them.
Les explains that in Acts 2 the Jewish crowd asked Peter and the rest of the apostles, “Men and brethren, what shall we do?” The pronoun is “we,” because God was dealing with the nation of Israel, so the question was appropriate: “What must we, the nation, do?” Peter answered, “Repent and be baptized every one of you in the name of Jesus Christ for the remissions of sins, and you shall receive the gift of the Holy Ghost.” Les emphasizes “every one,” noting the same statement appears again in chapter 3. The idea was that if every Jew, every Israelite, had done just that—repented of their national sin and recognized that the one they killed was indeed their Messiah and their King—chapter 3 tells us God would have sent Christ at that time to be their King and set up the kingdom. Les also reminds his class to look in these early chapters of Acts for death, burial, and resurrection as the means of salvation, but it is not there, so we have no right to put it there.