9 “Why do you baptize then, if you be not the Christ, nor Elias, nor a prophet? John answered them, saying, I baptize with water; but there stands One among you whom you know not.” For, very truly, He was not seen, being humble, and therefore was the lamp lighted. Observe how John gives place, who might have been accounted other than he was. “He it is who comes after me, who is made before me” (that is, as we have already said, is “preferred before me”), whose shoe's latchet I am not worthy to unloose.
How greatly did he humble himself! And therefore he was greatly lifted up; for he that humbles himself shall be exalted. Hence, holy brethren, you ought to note that if John so humbled himself as to say, “I am not worthy to unloose His shoe-latchet,” what need they have to be humbled who say, “We baptize; what we give is ours, and what is ours is holy.” He said, Not I, but He; they say, We. John is not worthy to unloose His shoe's latchet; and if he had said he was worthy, how humble would he still have been!
And if he had said he was worthy, and had spoken thus, “He came after me who is made before me, the latchet of whose shoe I am only worthy to unloose,” he would have greatly humbled himself. But when he says that he is not worthy even to do this, truly was he full of the Holy Spirit, who in such fashion as a servant acknowledged his Lord, and merited to be made a friend instead of a servant.
Source: Tractates on the Gospel of John (New Advent)