John 13:8
“You shall never wash my feet.” “What doest thou, Peter? Rememberest thou not those former words? Did you not say, 'Be merciful to Yourself,' and heardest thou not in return, 'Get behind Me, Satan'? Are you not even so sobered, but are you yet vehement?” “Yea,” he says, “for what is being done is a great matter, and full of amazement.” Since then he did this from exceeding love, Christ in turn subdues him by the same; and as there He effected this by sharply rebuking him, and saying, “You are an offense unto Me,” so here also by saying,
“If I wash you not, you have no part with Me.” What then says that hot and burning one?
Vehement in deprecation, he becomes yet more vehement in acquiescence; but both from love. For why said He not wherefore He did this, instead of adding a threat? Because Peter would not have been persuaded. For had He said, “Suffer it, for by this I persuade you to be humbleminded,” Peter would have promised it ten thousand times, in order that his Master might not do this thing. But now what says He? He speaks of that which Peter most feared and dreaded, the being separated from Him; for it is he who continually asks, “Where are You going?” Wherefore also he said, “I will give even my life for You.” And if, after hearing, “What I do you know not now, but you shall know hereafter,” he still persisted, much more would he have done so had he learned (the meaning of the action). Therefore said He, “but you shall know hereafter,” as being aware, that should he learn it immediately he would still resist. And Peter said not, “Tell me, that I may suffer You,” but (which was much more vehement) he did not even endure to learn, but withstands Him, saying, “You shall never wash my feet.” But as soon as He threatened, he straightway relaxed his tone. But what means, “You shall know after this”? “After this?” When? “When in My Name you shall have cast out devils; when you shall have seen Me taken up into Heaven, when you shall have learned from the Spirit that I sit on His right hand, then shall you understand what is being done now.” What then says Christ? When Peter said, “not my feet only, but also my hands and my head,” He replies,
Source: Homilies on the Gospel of John (New Advent)