Mark"s synopsis of John"s message is brief (cf. Matthew 3:7-10; Luke 3:10-14). It stresses the coming of the mighty One who would baptize with the Holy Spirit. John described the greatness of this One by contrasting himself with the Messiah. Slaves did not have to untie their masters" sandals, but John felt unworthy to do even this most menial task for Messiah. This emphasis on the humility of God"s servants persists through this Gospel. [source][source][source]
Another contrast is the baptisms of the two men ( Mark 1:8). This one shows the superior ministry of the Coming One. [source][source][source]
"The Baptist evidently meant that the great coming One would not merely cleanse with water but would bring to bear, like a deluge, the purging, purifying, judging presence of God himself." [1][source]
Jesus" baptism with the Holy Spirit probably looks forward to a baptism yet future from our viewpoint in history. In Matthew and in Luke"s account of this statement John said Jesus would baptize "with the Holy Spirit and fire." The single article before two nouns in the Greek text implies a single baptism with Spirit and fire. While such a baptism happened on the day of Pentecost (cf. Acts 1:5; Acts 2:32-33), not all of what the prophets predicted would happen when this baptism took place really transpired then (cf. Isaiah 44:3; Joel 2:28-32). Consequently we anticipate a future baptism with the Spirit and fire that will fulfill these prophecies completely. [source][source][source]