2 For thus she speaks: “I sleep, but my heart wakes: it is the voice of my Beloved that knocks at the gate.” And then He also says: “Open to me, my sister, my nearest, my dove, my perfect one; for my head is filled with dew, and my hair with the drops of the night.” And she replies: “I have put off my dress; how shall I put it on? I have washed my feet; how shall I defile them?” O wonderful sacramental symbol! O lofty mystery! Does she, then, fear to defile her feet in coming to Him who washed the feet of His disciples?
Her fear is genuine; for it is along the earth she has to come to Him, who is still on earth, because refusing to leave His own who are stationed here. Is it not He that says, “Lo, I am with you always, even unto the end of the world”? Is it not He that says, “You shall see the heavens opened, and the angels of God ascending and descending upon the Son of man”? If they ascend to Him because He is above, how do they descend to Him, but because He is also here? Therefore says the Church: “I have washed my feet; how shall I defile them?”
She says so even in the case of those who, purified from all dross, can say: “I desire to depart, and to be with Christ; nevertheless to abide in the flesh is more needful for you.” She says it in those who preach Christ, and open to Him the door, that He may dwell by faith in the hearts of men. In such she says it, when they deliberate whether to undertake such a ministry, for which they do not consider themselves qualified, so as to discharge it blamelessly, and so as not, after preaching to others, themselves to become castaways. For it is safer to hear than to preach the truth: for in the hearing, humility is preserved; but when it is preached, it is scarcely possible for any man to hinder the entrance of some small measure of boasting, whereby the feet at least are defiled.
Source: Tractates on the Gospel of John (New Advent)