Why did Peter in the early Acts period preach repentance and baptism?
Peter proclaimed repentance and baptism because he was still operating on covenant ground, not yet understanding the finished work of the cross. The early Acts period reflected that transitional condition, where the Holy Spirit followed repentance and baptism, before the later revelation of Paul's prison epistles.
Les explains that Peter did not yet realize the finished work of the cross. He was still on covenant ground, so he continued to proclaim a repentance and baptism, and then the Holy Spirit. Les contrasts this with Paul's later prison epistles, such as Ephesians, which come from his Roman imprisonment and reveal higher doctrine. In those later writings, Paul does not say "repented and baptized" but speaks of believers who first trusted in Christ.