the straps of whose sandals I am not worthy to untie: The untying of sandals was done only by slaves. John was showing great honor to Jesus the Messiah.
Baptize you with the Holy Spirit and fire: Old Testament prophets said God would give his Spirit to his people in the last times (Isaiah 32:15, Ezekiel 36:28–29; Joel 2:28). This prophecy partly came true on the day of Pentecost (Acts 2). Fire often represents judgment (see Luke 3:9). It can also represent the spiritual cleansing and purifying of God's people (see 1 Corinthians 3:10–15; compare Isaiah 4:4; Malachi 3:2).