5 Accordingly the Church, which Christ cleanses with the washing of water in the word, is without spot and wrinkle, not only in the case of those who are taken away immediately after the washing of regeneration from the contagious influence of this life, and tread not the earth so as to make necessary the washing of their feet, but in those also who have experienced such mercy from the Lord as to be enabled to quit this present life even with feet that have been washed.
But although the Church be also clean in respect of those who tarry on earth, because they live righteously; yet have they need to be washing their feet, because they assuredly are not without sin. For this cause is it said in the Song of Songs, “I have washed my feet; how shall I defile them?” For one so speaks when he is constrained to come to Christ, and in coming has to bring his feet into contact with the ground. But again, there is another question that arises. Is not Christ above? has He not ascended into heaven, and sits He not at the Father's right hand?
Does not the apostle expressly declare, “If you, then, be risen with Christ, set your thoughts on those things which are above, where Christ is sitting on the right hand of God. Seek the things which are above, not things which are on earth?” How is it, then, that to get to Christ we are compelled to tread the earth, since rather our hearts ought to be turned upwards toward the Lord, that we may be enabled to dwell in His presence? You see, brethren, the shortness of the time today curtails our consideration of this question.
And if you perhaps fail in some measure to do so, yet I for my part see how much clearing up it requires. And therefore I beg of you to suffer it rather to be adjourned, than to be treated now in too negligent and restricted a manner; and your expectations will not be defrauded, but only deferred. For the Lord who thus makes us your debtors, will be present to enable us also to pay our debts.
Source: Tractates on the Gospel of John (New Advent)