6 Knowing, then, these things, “He rises from supper, and lays aside His garments; and took a towel, and girded Himself. After that He pours water into a basin, and began to wash the disciples' feet, and to wipe them with the towel wherewith He was girded.” We ought, dearly beloved, carefully to mark the meaning of the evangelist; because that, when about to speak of the pre-eminent humility of the Lord, it was his desire first to commend His majesty. It is in reference to this that he says, “Jesus knowing that the Father had given all things into His hands, and that He has come from God, and is going to God.”
It is He, therefore, into whose hands the Father had given all things, who now washes, not the disciples' hands, but their feet: and it was just while knowing that He had come from God, and was proceeding to God, that He discharged the office of a servant, not of God the Lord, but of man. And this also is referred to by the prefatory notice he has been pleased to make of His betrayer, who was now come as such, and was not unknown to Him; that the greatness of His humility should be still further enhanced by the fact that He did not esteem it beneath His dignity to wash also the feet of one whose hands He already foresaw to be steeped in wickedness.
Source: Tractates on the Gospel of John (New Advent)