7 But perhaps he was not yet a Christian. Let us hear then, Brethren, to whom as believers the Gospel is read, by whom He who spoke these things, is worshipped, whose mark is borne by us on our forehead, and is held in the heart. For of very great concernment is it where a man has the mark of Christ, whether in the forehead, or both in the forehead and the heart. You have heard today the words of the holy prophet Ezekiel, how that before God sent one to destroy the ungodly people, He first sent one to mark them, and said to him, “Go and set a mark upon the foreheads of the men that sigh and moan for the sins of my people that are done in the midst of them.” He did not say, “which are done without them;” but “in the midst of them.”
Yet they “sigh and moan;” and therefore are they “marked on the forehead:” in the forehead of the inner man, not the outer. For there is a forehead in the face, there is a forehead in the conscience. So it happens that when the inner forehead is stricken, the outer grows red; either red with shame, or pale with fear. So then there is a forehead of the inner man. There were they “marked” that they might not be destroyed; because though they did not correct the sins which were “done in the midst of them,” yet they sorrowed for them, and by that very sorrow separated themselves; and though separated in God's sight, they were mixed with them in the eyes of men.
They are “marked” secretly, are not hurt openly. Afterwards the Destroyer is sent, and to him it is said, “Go, lay waste, spare neither young nor old, male nor female, but come not near those who have the mark on their forehead.” How great security is granted to you, my Brethren, who among this people are sighing, and moaning for the iniquities which are being done in the midst of you, and who do them not!
Source: Sermons on Selected Lessons of the New Testament (New Advent)