8 “Then there arose a question on the part of John's disciples with the Jews about purifying.” John baptized, Christ baptized. John's disciples were moved; there was a running after Christ, people were coming to John. Those who came to John, he sent to Jesus to be baptized; but they who were baptized by Christ were not sent to John. John's disciples were alarmed, and began to dispute with the Jews, as usually happens. Understand the Jews to have declared that Christ was greater, and that to His baptism people ought to have recourse.
John's disciples, not yet understanding this, defended John's baptism. They came to John himself, that he might solve the question. Understand, beloved. And here we are given to see the use of humility, and, when people were erring in the subject of dispute, are shown whether John desired to glory in himself. Now probably he said, “You say the truth, you contend rightly; mine is the better baptism, I baptized Christ Himself.” John could say this after Christ was baptized. If he wished to exalt himself, what an opportunity he had to do so!
But he knew better before whom to humble himself: to Him whom he knew to have come after himself by birth, he willingly yielded precedence by confessing Him. He understood his own salvation to be in Christ. He had already said above, “We all have received out of His fullness;” and this is to confess Him to be God. For how can all men receive of His fullness, if He be not God? For if He is man in such wise that He is not God, then Himself also receives of the fullness of God, and so is not God.
But if all men receive of His fullness, He is the fountain, they are drinkers. They that drink of a fountain, both thirst and drink. The fountain never thirsts; it has never need of itself. Men need a fountain. With thirsty stomachs and parched lips they run to the fountain to be refreshed. The fountain flows to refresh, so does the Lord Jesus.
Source: Tractates on the Gospel of John (New Advent)