4 “If I, then,” He says, “your Lord and Master, have washed your feet, you also ought to wash one another's feet. For I have given you an example, that you should do as I have done to you.” This, blessed Peter, is what thou did not know when thou were not allowing it to be done. This is what He promised to let you know afterwards, when your Master and your Lord terrified you into submission, and washed your feet. We have learned, brethren, humility from the Highest; let us, as humble, do to one another what He, the Highest, did in His humility.
Great is the commendation we have here of humility: and brethren do this to one another in turn, even in the visible act itself, when they treat one another with hospitality; for the practice of such humility is generally prevalent, and finds expression in the very deed that makes it discernible. And hence the apostle, when he would commend the well-deserving widow, says, “If she is hospitable, if she has washed the saints' feet.” And wherever such is not the practice among the saints, what they do not with the hand they do in heart, if they are of the number of those who are addressed in the hymn of the three blessed men, “O you holy and humble of heart, bless ye the Lord.” But it is far better, and beyond all dispute more accordant with the truth, that it should also be done with the hands; nor should the Christian think it beneath him to do what was done by Christ.
For when the body is bent at a brother's feet, the feeling of such humility is either awakened in the heart itself, or is strengthened if already present.
Source: Tractates on the Gospel of John (New Advent)