I baptize you with water for repentance, but he who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire.
John the Baptist had a sharp edge to his short and powerful ministry. He was the last of those who looked and felt like Old Testament prophets. He drew people to the desert in multitudes to repent of sin and be baptized. He prepared the way for Jesus. He preached about the coming Messiah as a person of power and judgment.
John saw the power of Messiah imminent not in political action, but as the One Who will baptize with the Holy Spirit. The Messiah would be leading a ministry that would immerse believers in the Spirit of God. Clearly this was more than just a temporal deliverance. John was preaching of a salvation that would come in Jesus.
John the Baptist also pointed to a Messiah Who would have the power to judge sin rightly. He would purge with fire (a definitive work exclusively the domain of God as any reader of the Old Testament prophets would know) and would purify His people again. The world would come into contact with God's justice in the Messiah. Jesus would also be the only judge.
Of course Jesus is both Holy God and the refreshing giver of the Holy Spirit. He brings the baptisms of the Holy Spirit and of fire. The first came upon His disciples at Pentecost and continues in each believer upon salvation. The second is found in His authority to judge and Jesus will come again on the white horse of Judgment with a fire that will purge this universe. Jesus is the giver of life and the judge of all men. He is mightier than I.