Acts 2:29-36

²⁹"Brothers, I can tell you confidently that the patriarch David died and was buried, and his tomb is here to this day. ³⁰But he was a prophet and knew that God had promised him on oath that he would place one of his descendants on his throne. ³¹Seeing what was ahead, he spoke of the resurrection of the Christ, that he was not abandoned to the grave, nor did his body see decay. ³²God has raised this Jesus to life, and we are all witnesses of the fact. ³³Exalted to the right hand of God, he has received from the Father the promised Holy Spirit and has poured out what you now see and hear. ³⁴For David did not ascend to heaven, and yet he said,
" 'The Lord said to my Lord:
"Sit at my right hand
³⁵until I make your enemies
a footstool for your feet."

³⁶"Therefore let all Israel be assured of this: God has made this Jesus, whom you crucified, both Lord and Christ."

Jesus is Lord & Christ

Peter, in all his boldness, explained to the Jews in attendance what just had happened. The Holy Spirit was poured out upon the apostles that day before their very eyes. It was awesome and yet confusing. That is why Peter spoke about the phophecy of the patriarch David, so they would listen and understand that in fact Jesus was the one who fulfilled these very scriptures that they were familiar with.