John Chrysostom
That this dispute arose from envy is clear from the events that follow, which the Evangelist also sets forth without reproach. "And they came," he says, "to John and said to him: Rabbi, He who was with you beyond the Jordan, to whom you have borne witness, behold, He is baptizing, and all are going to Him."
That is, they mean, to the One whom you baptized. They express this by the words, "to whom you have borne witness."
They were speaking as though to say: The One whom you made known and illustrious has dared to do the same thing that you do.
But they did not say, "whom you baptized." In that case they would have been forced to remember the voice from above and the descent of the Spirit.
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What then do they say? "He who was with you beyond the Jordan, to whom you have borne witness." That is, He who was among your disciples, who had nothing more in Himself than we have, has separated from you and is baptizing.
They thought to stir John not only by this, but also by saying that their glory was now being overshadowed. "All," they say, "are going to Him."
From this it is clear that they had not overcome even the Jew with whom they had disputed. They spoke in this way because they were still imperfect and not yet cleansed of vainglory.