1 I Have not been unmindful of my debt, and acknowledge that the time of payment has now come. May He give me wherewith to pay, as He gave me cause to incur the debt. For He has given me the love, of which it is said, “Owe no man anything, but to love one another.” May He give also the word, which I feel myself owing to those I love. I put off your expectations till now for this reason, that I might explain as I could how it is we come to Christ along the ground, when we are commanded rather to seek the things which are above, not the things which are upon the earth. For Christ is sitting above, at the right hand of the Father: but He is assuredly here also; and for that reason said also to Saul, as he was raging on the earth, “Why do you persecute me?” But the topic on which we were speaking, and which led to our entering on this inquiry, was our Lord's washing His disciples' feet, after the disciples themselves had already been washed, and needed not, save to wash their feet.
And we there saw it to be understood that a man is indeed wholly washed in baptism; but while thereafter he lives in this present world, and with the feet of his human passions treads on this earth, that is, in his life-intercourse with others, he contracts enough to call forth the prayer, “Forgive us our debts.” And thus from these also is he cleansed by Him who washed His disciples' feet, and ceases not to make intercession for us. And here occurred the words of the Church in the Song of Songs, when she says, “I have washed my feet; how shall I defile them?” when she wished to go and open to that Being, fairer in form than the sons of men, who had come to her and knocked, and asked her to open to Him.
This gave rise to a question, which we were unwilling to compress into the narrow limits of the time, and therefore deferred till now, in what sense the Church, when on her way to Christ, may be afraid of defiling her feet, which she had washed in the baptism of Christ.
Source: Tractates on the Gospel of John (New Advent)