4 But what is this? What does it mean? And what is there in it we need to examine? The Lord says, The Truth declares that even he who has been washed has need still to wash his feet. What, my brethren, what think you of it, save that in holy baptism a man has all of him washed, not all save his feet, but every whit; and yet, while thereafter living in this human state, he cannot fail to tread on the ground with his feet. And thus our human feelings themselves, which are inseparable from our mortal life on earth, are like feet wherewith we are brought into sensible contact with human affairs; and are so in such a way, that if we say we have no sin, we deceive ourselves, and the truth is not in us. And every day, therefore, is He who intercedes for us, washing our feet: and that we, too have daily need to be washing our feet, that is ordering aright the path of our spiritual footsteps, we acknowledge even in the Lord's prayer, when we say, “Forgive us our debts as we also forgive our debtors.” For “if,” as it is written, “we confess our sins,” then verily is He, who washed His disciples' feet, “faithful and just to forgive us our sins, and to cleanse us from all unrighteousness,” that is, even to our feet wherewith we walk on the earth.
Source: Tractates on the Gospel of John (New Advent)