John the Baptist was not the greatest man on earth, but he was the greatest figure in redemptive revelation up to that time—not morally greater, but greater in his ministry as the herald of the Messiah (Matthew 11:10), even in spite of his doubts (Matthew 11:2–3).
No, in Matthew 11:11 Jesus simply proclaims that John was the greatest figure in redemptive-revelational history up to that point—greater even than Moses, David, and Isaiah. This does not mean that he is comparing people with one another in a moral sense, but he is stating that John was the greatest precisely because of his ministry as the forerunner of the Messiah (Matthew 11:10). Not even his doubts expressed in Matthew 11:2–3 diminished his greatness or his ministry in the eyes of Jesus.1
11 Truly, I say to you, among those born of women there has arisen no one greater than John the Baptist. Yet the one who is least in the kingdom of heaven is greater than he.