“Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified.” (Acts 2:36 ESV)
Peter’s explanation of the miracles of Pentecost, particularly the speaking in tongues, gave him an opportunity to preach the gospel. This short sermon contains the basic facts of Jesus ministry, death and resurrection. These facts are enhanced by an appeal to the witness of the Old Testament Scriptures, in this case the prophet Joel and King David. Peter also cited the eyewitness testimony of the apostles and the testimony of the people who were there listening.
The important idea that Peter wanted to convey to those listeners was that now is the day of salvation, now is man made right with God. This is possible only by the death and resurrection of Jesus, the very man that the Jews condemned and rejected several weeks earlier. Peter let them know that Jesus had risen from the dead. He was no ordinary man, but He is Lord, that is Yahweh, as well as the promised Messiah. In the ensuing verses Peter called them to acknowledge their sins, repent and be baptized. Then they would receive forgiveness, enter into the new life in the Kingdom of God and filled by the promised Holy Spirit and in fellowship of the new community of God, the church. All who are saved are rooted in that fellowship of believers. It is impossible to be a Christian without the fellowship of the church for love and faith need fellowship in order to develop and function.