Peter reached the climax of his lesson: God had made both Lord and Christ the Jesus whom they had crucified. 

They heard it and were cut to the heart; they wanted to know what they were to do. 

Peter told them to repent and be baptized for the remission of their sins, and they would receive the gift of the Holy Spirit.  The promise of the Holy Spirit would be given to them and to all afar off whom God called to Himself.