5 But apart from this moral understanding of the passage, we remember that the way in which we commended to your attention the grandeur of this act of the Lord's, was that, in washing the feet of disciples who were already washed and clean, the Lord instituted a sign, to the end that, on account of the human feelings that occupy us on earth, however far we may have advanced in our apprehension of righteousness, we might know that we are not exempt from sin; which He thereafter washes away by interceding for us, when we pray the Father, who is in heaven, to forgive us our debts, as we also forgive our debtors. What connection, then, can such an understanding of the passage have with that which He afterwards gave Himself, when He explained the reason of His act in the words, “If I then, 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”? Can we say that even a brother may cleanse a brother from the contracted stain of wrongdoing? Yea, verily, we know that of this also we were admonished in the profound significance of this work of the Lord's, that we should confess our faults one to another, and pray for one another, even as Christ also makes intercession for us. Let us listen to the Apostle James, who states this precept with the greatest clearness when he says, “Confess your faults one to another, and pray one for another.” For of this also the Lord gave us the example.
For if He who neither has, nor had, nor will have any sin, prays for our sins, how much more ought we to pray for one another's in turn! And if He forgives us, whom we have nothing to forgive; how much more ought we, who are unable to live here without sin, to forgive one another! For what else does the Lord apparently intimate in the profound significance of this sacramental sign, when He says, “For I have given you an example, that you should do as I have done to you;” but what the apostle declares in the plainest terms, “Forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also do ye”? Let us therefore forgive one another his faults, and pray for one another's faults, and thus in a manner be washing one another's feet.
It is our part, by His grace, to be supplying the service of love and humility: it is His to hear us, and to cleanse us from all the pollution of our sins through Christ, and in Christ; so that what we forgive even to others, that is, loose on earth, may be loosed in heaven.
Source: Tractates on the Gospel of John (New Advent)