He quotes language of the prophets to prove this. He then presents the testimony of himself and his fellow apostles to the effect that Jesus had been raised from the dead, and that they had seen him with their eyes and handled him with their hands. He further states that God had said to Jesus: "Sit thou on my right hand, till I make thine enemies the footstool of thy feet," and closes this powerful argument with this soul-stirring appeal: "Let all the house of Israel therefore know assuredly that God hath made him both Lord and Christ, this Jesus whom ye crucified."
Three thousand of those who stood in the hearing of Peter's voice believed this, felt pricked in their hearts--that sense of guilt which overwhelmed them when they realized that they had been guilty of murdering the Son of the living God, the greatest crime that human being ever committed--and in great agony of soul, they cried out: "Brethren, what shall we do?" to get rid of this pricking of our hearts, to get rid of the awful crime, to get rid of our sins before God and escape its consequences in the day of God's wrath against sin.
Moved by the Holy Spirit, Peter answered: "Repent ye, and be baptized, every one of you, in the name of Jesus Christ unto the remission of your sins; and ye shall receive the gift of the Holy Spirit." This was God's answer, that enabled them to get rid of their guilt and condemnation at once. And to assure them still further, he said: "For to you is the promise" (the remission of sins and the gift of the Holy Spirit), "and to your children, and to all that are afar off, even as many as the Lord our God shall call unto him," for the commission was to "all nations," "even unto the end of the world." But Peter did not stop here, for "with many other words he testified, and exhorted them, saying, 'Save yourselves from this crooked generation.' Then they that received his word were baptized; and there were added unto them in that day about three thousand souls."
Now, let us see if we can gather from this brief narrative what agencies God used in bringing about the conversion of these people, and what conditions they had to comply with in order to receive the benefits of the redemption which was provided by the blood of Christ.