"I baptize with water," John replied, "but among you stands one you do not know. He is the one who comes after me, the thongs of whose sandals I am not worthy to untie.""
Water baptism is only the sign of Spirit baptism, just as animal sacrifices were only a sign of the sacrifice of the Messiah. John was telling them that the Messiah was already among them and they had not recognized him. Among the people John already had a fine reputation. They were talking of him as a new Elijah or a new prophet like Isaiah or Jeremiah or Daniel. John on the other hand knows that he is a little man, minuscule next to Jesus.
There is a famous comedy sketch by John Cleese, Ronnie Barker and Ronnie Corbett, who represented different British classes. The tall Cleese says, "I'm upper class; I look down on these." The medium sized Barker says, "I'm middle class; I look up to him, but I look down on him." The short (tiny even) Corbett simply says, "I know my place."
John the Baptist knew his place. Do we know ours? I saw an interview with a young person on TV yesterday. When asked what interested her most in the world; what she gave most importance to, she replied, "Myself." No-one chiefly interested in themselves can enter into the kingdom of heaven. You'd be surprised how many supposed Christians would have the same interest.