This is how we commended the deep meaning of this deed of the Lord: by washing the feet of his already washed and clean disciples, the Lord provided a sign, on account of the human feelings that we have on earth, that however much progress we have made in achieving righteousness, we should recognize that we are not without sin, sin that he then washes away by interceding for us when we pray the Father who is in heaven to forgive us our debts as we forgive our debtors (Mt 6:12).
How does this understanding relate to what he himself said later when he explained what he had done by saying, “If, then, I, Lord and Master, have washed your feet, you too must wash one another’s feet. I have given you an example so that what I have done for you, you also must do.” Can we say that one brother can cleanse another from the infection of sin? Yes. We know that to this we were admonished in the deep significance of this deed of the Lord’s, that, having confessed our sins to one another we should pray for ourselves just as Christ intercedes for us (Rom 8:34). Let us listen to the Apostle James when he very clearly commands this: “Confess your sins to one another, and pray for one another” (Jas 5:16). For the Lord gave us an example of this too. For if he who has no sin, had no sin, and will have no sin prays for our sins, how much more should we pray for one another’s sin! And if he who has nothing we must forgive forgives us, how much more should we forgive one another, we who are not able to live here without sin!
For when the Lord says, “I have given you an example that what I have done for you, you also should do,” what else does he seem to be signifying in the depths of this mystery than what the Apostle says most openly: “Forgiving one another if anyone has a quarrel with another, just as the Lord forgave you, so also you must forgive” (Col 3:13). Let us forgive one another’s sins, then, and pray for one another’s sins and in this way wash one another’s feet. Our part is, by his gift, to perform this ministry of charity and humility; his is to hear us and to cleanse use from all the contamination of our sins through Christ and in Christ so that what we forgive to others, what we loose on earth, may be loosed in heaven. (In Ioannem Tr. 58, 5; PL 35, 1794-1795)