Jerusalem was the center of the 3 Jewish feasts, one of them was the Feast of Pentecost, 50 days after the Passover. Acts 2 says that 10 days after the ascension of Jesus, foreign Jews from many places are in Jerusalem to celebrate Pentecost. In this occasion, the Holy Spirit came over the disciples and made them talk in other languages, attracting the attention of the foreigner Jews. Peter explained that they were in a very special moment in history - when prophecies are being fulfilled. He uses Psalm 16 as a prophecy to the resurrection of the Messiah, and Joel 2 to explain the coming of the Holy Spirit. Peter explained that Jesus is at the right or the Lord waiting for his enemies to under His feet as in Psalm 110. Peter said:
36 Therefore let all the house of Israel know for certain that God has made Him both Lord and Christ — this Jesus whom you crucified.”
Most of the listeners should have been 50 days before in the Passover when Jesus was crucified by their approval. Thus, given that everything Peter was saying were true, they became the enemies of the Lord. The logical question they made was: what can we do? Peter said:
8 Peter said to them, “Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit.
Let this gift of the Holy Spirit be with me, changing my life so that I become more and more like Jesus.