“Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all.”
Jesus said to him, 'One who has bathed does not need to wash, except for the feet, but is entirely clean. And you are clean, but not all of you' — the distinction between bathing (complete immersion) and washing feet is crucial: the disciples have been bathed in the word Jesus has spoken, sanctified by encounter with him, yet they walk through the world and gather dust. The footwashing is not repeated baptism but ongoing purification from the world's contamination, a sign of present grace in the midst of their incomplete journey. The ominous final clause—"not all of you"—introduces Judas obliquely, the one whose feet are washed yet whose heart remains unwashed, showing that external cleansing cannot of itself transform intention or choice.
Church Fathers on John 13:10
As a physician, who has many sick under his care, begins with those who want his attention most, so Christ, in washing His disciples’ feet, begins with the most unclean, and so comes at last to Peter, who needed the washing less than any: Then comes He to Simon Peter. Peter resisted being washed, perhaps because his feet were nearly clean: and Peter said to Him, Lord, do you wash my feet?
What is the meaning of you and my feet? It is better to think than speak of this; lest one should fail in explaining adequately what might have been rightly conceived.
Though Peter was the first of the Apostles, yet it is possible that the traitor petulantly placed himself above him; and that this may be the reason why our Lord first began to wash, and then comes to Peter.