The out-pouring of the Holy Spirit started something that has changed the world. After the full, powerful and alarming sound of tornados that filled Jerusalem, men from everywhere came to see what this noise was and what it "meant."
With his audience so conveniently gathered before him, Peter began to present Jesus to Israel as her Messiah. Joel's prophecy was being fulfilled in front of their eyes! It gave Peter an opening to discuss two critical points: Who was the "Lord" on whom they should call for deliverance from coming judgment AND What should they do.
After Peter powerfully proved to the gathered multitudes that Jesus was the Lord and Christ whom all Israel expected and desired, they cried out, "What must we do?"
We'll see how Peter convinced them of point #1 and what he commanded them regarding point #2..