John 13:7

[Our Lord teaches] that this act was a mystery. But what was it that Jesus was doing when he washed the disciples' feet? Was he, by washing their feet and drying them with the towel with which he had girded himself, making them beautiful, since they were about to preach the good news?…Now the feet of those proclaiming good news became beautiful, so that, when they were washed and cleansed and dried by Jesus' hands, they might be able to walk on the holy way and travel over him who said, "I am the way." For he alone, and everyone who has had his feet washed by Jesus, travels over this way, which is living and which brings one to the Father. This way admits no feet that are defiled and not yet clean. Moses therefore had to loose the sandals from his feet since the place to which he had come, on which he stood, was holy ground. And the same was true of Joshua the son of Nun. …
For me to wash your feet [Jesus says] is symbolic of the bases of your souls being purified, that they may be beautiful, since you are to preach the good news and to approach the souls of people with your feet clean. But you do not now know this mystery, inasmuch as you do not yet have knowledge of it. Such knowledge will more appropriately be present in you when I have washed your feet, and after this you will know, when you understand this mystery and are enlightened.

Let us come to the Gospel. We find there our Lord and Savior "putting off his garments, and, girded with a linen cloth, pouring water into a basin, and washing the feet of his disciples." You see, then, that the prophets foreshadowed these things, which were to be fulfilled in the last times through the Lord. That "water," therefore, which "Jesus poured into the basin," was the dew of heavenly grace, with which "he washed the feet of his disciples." Whence also he rightly said to them: "but you are clean because of the word which I have spoken to you." But we too, if only we offer our feet, the Lord Jesus is ready to "wash the feet" of our soul and to cleanse them with the heavenly dew, with the grace of the Holy Spirit, with the word of doctrine. For he willed that not the Apostles alone should be "clean," but also all who believe through his word. And to all believers he says that which he said to Peter: "unless I wash you, you shall have no part with me." For it is certain that no one "has part with Christ" unless he has been washed and made "clean." Come, I beseech you, Lord Jesus, Son of God, "put off the garments" which you put on for my sake, and gird yourself for my sake, and "pour water into a basin and wash the feet" of your servants, wash away the filth of your sons and daughters. "Wash the feet" of our soul, that we, imitating you and following you, may "put off" our old "garments." For as soon as you "wash" my "feet," may you also make me recline with you, that I may hear from you: "you call me Lord and master; and you say rightly, for I am. If therefore I, the Lord and master, have washed your feet, you too wash one another's feet." I too, therefore, now wish to "wash the feet" of my brethren, to "wash the feet" of my fellow-disciples. And so far as I am able, I desire to "wash the feet" of my brethren and to fulfill the commandment of the Lord, that by the word of doctrine the hearers may be cleansed from the filth of their sins.

"He saith unto him, What I do thou knowest not now, but thou shall know here after."

That is "thou shall know how great is the gain from this, the profit of the lesson, and how it is able to guide us into all humblemindedness." What then doth Peter? He still hinders Him, and saith, "Thou shall never wash my feet." "What doest thou, Peter? Rememberest thou not those former words? Saidst thou not, 'Be merciful to Thyself,' and heardest thou not in return, 'Get thee behind Me, Satan'? Art thou not even so sobered, but art thou yet vehement?" "Yea," he saith, "for what is being done is a great matter, and full of amazement." Since then he did this from exceeding love, Christ in turn subdueth him by the same; and as there He effected this by sharply rebuking him, and saying, "Thou art an offense unto Me," so here also by saying, "If I wash thee not, thou hast no part with Me."

Jesus answered and said unto him, What I do thou knowest not now; but thou shalt know hereafter.

But "Jesus answered and said unto him, What I do thou knowest not now, but thou shalt know hereafter." And not even yet, terrified as he was by the sublimity of the Lord's action, does he allow it to be done, while ignorant of its purpose; but is unwilling to see, unable to endure, that Christ should thus humble Himself to his very feet.

Inasmuch therefore as He had come to what manifestly and obviously is the central point of the incident before us, He says: "If thou shouldst refuse to receive this strange and novel lesson of humility, thou wouldst find no part or lot with Me." And since oftentimes our Lord Jesus the Christ, taking small matters as the suggestive occasions of His discourses, makes His exposition of general application; and, drawing out to a wide range the lessons arising out of a single event or the words spoken solely with regard to some individual circumstance, introduces into the discussion of the matters in hand a rich abundance of profitable illustrations: we shall suppose that in this also He meant to say that unless through His grace a man washes away from himself the defilement of sin and error, he will have no share in the life that proceeds from Him, and will remain without a taste of the kingdom of heaven. For the uncleansed may not enter the mansions above, but only they who have their conscience cleansed by love to Christ, and have been sanctified in the Spirit by Holy Baptism

A brother asked a hermit, 'Abba, look here, I ask my elders questions, and they talk to me for the good of my soul, and I remember nothing they say. Is it any use asking questions when I gain nothing by it? I am deeply sinful.' There were two empty vessels nearby. The hermit said, 'Take one of those vessels and put oil in it, rinse it, pour out the oil, and bring the vessel back.' He did so. He said, 'Do it again.' He did so. After he had done it several times, the hermit said, 'Now, take both vessels and see which is the cleaner.' He answered, 'The one into which I put oil.' The hermit said, 'It's the same for the one who asks questions. Although you remember nothing that you have heard, your soul will be cleaner than that of someone who never even asks questions.'

What does the Lord say? He says: "What I am doing, you do not know now; My deed contains within it a profound humility, which I am also teaching you. Yet afterward, when you will cast out demons in My name, when you see My ascension into heaven, when through the Holy Spirit you learn that I sit at the right hand of the Father, then you will understand that He who humbled Himself to the point of washing your feet is the very same One who gives you such power over demons and who ascended and was glorified with the Father, having been in no way diminished by His humility. Therefore, you too must implant within yourself humility, which does not diminish but rather exalts."

Jesus said to him: What I do you do not know now, but you shall know hereafter; therefore it is fitting that you endure until later. But Peter is not swayed by the admonition.

Then (v 7), we see the words of Christ, which show that this action is a mystery. Christ said to Peter: What I am doing you do not know now, but afterward you will understand. This action is both an example and a mystery. It is an example of humility to be practiced: "For I have given you an example, that you also should do as I have done to you" (v 15) And it is a mystery because it signifies an interior cleansing: "He who has bathed does not need to wash, except for his feet" (v 10).

So what Christ said can be understood in two ways. In one way, What I am doing you do not know now, that is, you do not now understand that what I am doing is an example; but afterward you will understand, when he explained it to them saying: "Do you know what I have done to you?" (v 12). In another way, What I am doing you do not know now; that is to say, this is a mystery and something hidden, and it signifies an interior cleansing which only I can accomplish, and which you do not understand now, but afterward you will understand, when you receive the Holy Spirit: "I have yet many things to say to you, but you cannot bear them now. When the Spirit of truth comes, he will guide you into all the truth" (16:12).