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.
Matthew 3:11 ESV

John the Baptist understood God's plan of salvation and sanctification more clearly than any prophet who came before him. Even many great men of God who came later didn't understand their own history as well as John understood his future. See Acts 18:25.