Before proceeding into chapter two, I think that it would be a good time to give a primer on Pentecost. Firstly, in my opinion, Pentecost has everything to do with the Kingdom and nothing to do with the Body of Christ. I say this because the Body of Christ could not have begun until the Kingdom had been officially been offered and rejected by the nation of Israel. For too long I believed that this was done in the Gospels. No, it wasn't. In the Gospels, the message was "repent for the kingdom of heaven is at hand" (Matthew 3:2; 4:17; 10:7). Notice, it says, "at hand", not here, but at hand. It could not have been offered until the Messiah had been crucified (Hebrews 9:16-17). This is exactly what happened in the first several chapters of Acts. It was offered by Peter and was ultimately rejected at the stoning of Stephen. Afterward, the mystery of the Body of Christ was revealed to the Apostle Paul.
Again, the Gospel of the Kingdom had to be rejected first. Paul explains it this way in Romans 11:28-32, "As concerning the gospel, they (Jews) are enemies for your (Gentiles) sakes: but as touching the election, they (Jews) are beloved for the fathers' sakes. For the gifts and calling of God are without repentance. For as ye (Gentiles) in times past have not believed God, yet have now obtained mercy through their (Jews) unbelief: Even so have these (Jews) also now not believed, that through your (Gentiles) mercy they (Jews) also may obtain mercy. For God hath concluded them (Jews) all in unbelief, that he might have mercy upon all." Pentecost was the beginning of the Jewish nation being "in unbelief". The NASB uses the phrase "shut up in disobedience". This had to happen before mercy could be extended to the Gentile.
However, with that said, God was not surprised by their rejection and already had a plan that involved the Gentiles. Paul says in this way in Romans 11:15, "For if the casting away of them (Jews) be the reconciling of the world, what shall the receiving of them (Jews) be, but life from the dead?" In other words, the casting away of the Jews meant the reconciliation of the world. That means that it was only by the setting aside of Israel that the rest of the world could be a recipient of God’s grace through Christ's crucifixion. Of course, it will also be the removal of the church through which Israel will be the recipient of God’s promises to Abraham.