One more powerful than I: Even though he was popular and many people continued to follow him, John knew he was less important than the Messiah.
whose sandals I am not worthy to carry: In Jewish culture, removing and carrying sandals was a task for slaves. Not even a rabbi's disciple would do it.
with the Holy Spirit and with fire: This happened at Pentecost (see Joel 2:28–29; Acts 2:1–47). This may include the ministry of Jesus, empowered by the Holy Spirit (see Matthew 12:28; Luke 4:14). Fire could mean purification (Zech 13:9) or judgment (Malachi 3:2–3; 4:1; see Isaiah 1:25; 4:4). It is a “Spirit-and-fire baptism.” The Messiah completes what John could only start (full judgment and purification).