Why they who come forth from the laver of baptism are anointed on the head; why, too, after baptism, their feet are washed, and what sins are remitted in each case.
29. After this, you went up to the priest, consider what followed. Was it not that of which David speaks: “Like the ointment upon the head, which went down to the beard, even Aaron's beard”? This is the ointment of which Solomon, too, says: “Your Name is ointment poured out, therefore have the maidens loved You and drawn You.” How many souls regenerated this day have loved You, Lord Jesus, and have said: “Draw us after You, we are running after the odour of Your garments,” that they might drink in the odour of Your resurrection.
30. Consider now why this is done, for “the eyes of a wise man are in his head;” therefore the ointment flows down to the beard, that is to say, to the beauty of youth; and therefore, Aaron's beard, that we, too, may become a chosen race, priestly and precious, for we are all anointed with spiritual grace for a share in the kingdom of God and in the priesthood.
31. You went up from the font; remember the Gospel lesson. For our Lord Jesus Christ in the Gospel washed the feet of His disciples. When He came to Simon Peter, Peter said: “You shall never wash my feet.” He did not perceive the mystery, and therefore he refused the service, for he thought that the humility of the servant would be injured, if he patiently allowed the Lord to minister to him. And the Lord answered him: “If I wash not your feet, you will have no part with Me.” Peter, hearing this, replies: “Lord, not my feet only, but also my hands and my head.” The Lord answered: “He that is washed needs not save to wash his feet but is clean every whit.”
32. Peter was clean, but he must wash his feet, for he had sin by succession from the first man, when the serpent overthrew him and persuaded him to sin. His feet were therefore washed, that hereditary sins might be done away, for our own sins are remitted through baptism.
33. Observe at the same time that the mystery consists in the very office of humility, for Christ says: “If I, your Lord and Master, have washed your feet; how much more ought you to wash one another's feet.” For, since the Author of Salvation Himself redeemed us through His obedience, how much more ought we His servants to offer the service of our humility and obedience.
Source: On the Mysteries (New Advent)