Peter's first sermon at Pentecost transformed a once-fearful disciple into a bold proclaimer of truth. After the Holy Spirit descended upon believers, Peter addressed the crowd who witnessed them speaking in foreign languages. He explained that Jesus' ministry was divinely authenticated through miracles, his crucifixion fulfilled God's plan, and his resurrection, ascension, and glorification confirmed his identity as Lord and Messiah. This simple, Christ-centered message cut to the hearts of listeners, leading 3,000 people to repent, be baptized, and receive the Holy Spirit in a single day.