Harry Ironside emphasizes the ministry of Peter, highlighting the central theme of repentance in his preaching. He explains that Peter, as a key figure in the early church, consistently called for repentance and faith in Jesus Christ, linking it to the fulfillment of Old Testament prophecies. Ironside illustrates how Peter's messages at Pentecost and after the healing of the lame man underscored the necessity of changing one's attitude towards sin and God to receive forgiveness. The sermon stresses that repentance is not merely an emotional experience but a critical step towards salvation, applicable to both Jews and Gentiles. Ultimately, Ironside calls for preachers today to boldly proclaim the need for repentance as part of the Gospel message.