Ezekiel 9:4

But maybe that person wasn't yet a Christian. We at least, brothers and sisters, should listen; we to whom as believers the gospel is chanted, we by whom the one who said all this is worshiped, whose sign is worn by us on our foreheads and held in our hearts. It makes a great deal of difference, you see, where a person keeps the sign of Christ, whether on the forehead or both on the forehead and in the heart. You heard, when the holy prophet Ezekiel was speaking, how before God sent an exterminator of a wicked people, he first sent a marker and said to him, "Go and mark with a sign the foreheads of those who groan and grieve over the sins of my people, which are committed among them." Yet for all that they groan and grieve; and this is why they have been marked with a sign on the forehead—the forehead of the inner self, not the outer one. There is a forehead of the face, you see, and a forehead of the conscience. In fact, sometimes the inner forehead gets a knock, and the outer one blushes; it either blushes for shame or turns pale with fright.

(Verse 4 and following) And the Lord said to him: Go through the middle of the city, in the midst of Jerusalem, and put a mark on the foreheads of the men who groan and lament over all the abominations that are done in the midst of it. And to the others he said while I was listening: Go through the city after him and strike. Your eye shall not spare, nor shall you show pity. Old men, young men and maidens, little children and women, kill them all to total destruction. But on whomsoever you shall see Thau, ye shall not kill him: and begin ye at my sanctuary. So they began with the ancient men who were before the house. Or, as the Hebrews understand it, because the Law is called Torah among them, which is written with the letter at the beginning of its name; those who have fulfilled the precepts of the Law received this sign. And to come to our point, the ancient Hebrew letters, which the Samaritans still use today, have the shape of the letter Thau at the end, which bears resemblance to a cross, which is painted on the foreheads of Christians and is frequently marked in handwriting. There are those who think that, because in the Hebrew alphabet this is the last letter, it is demonstrated that the remains of the saints survive in a multitude of sinners. Therefore, those who groan and lament are saved; those who not only did not consent to evil deeds, but also mourned the sins of others, just as Samuel mourned over Saul (3 Kings 16), and the apostle Paul mourned over those who did not repent after sin. And he himself also said: We who are in this tabernacle groan (2 Cor. 5:2). And elsewhere: I have great sorrow and unceasing pain in my heart (or in my mind) (Al. cordi meo). […]

At the beginning of our prayers let us sign ourselves with the seal of baptism. Let us make the sign of the cross on our foreheads, as on the day of our baptism, as it is written in Ezekiel. Let us not first lower our …

God says that only those can escape who have been reborn and signed with the sign of Christ; when sending his angels to lay waste the world and to destroy the human race he threatens more seriously than the last time.… This sign pertains to the passion and blood of Christ and that he is kept safe and unharmed whoever is found in this sign.

And I have heard many, after such experience, blame themselves, and say, What advantage is it that I have grieved? I have not recovered my money, and I have injured myself. But if thou hast grieved on account of sin, thou hast blotted it out, and hast reaped the greatest pleasure. If thou hast grieved for thy brethren who have fallen, thou hast both encouraged and comforted thyself, and hast also restored them; and even if thou wert not to profit them, thou hast an abundant recompense. And that thou mayest learn that this grieving for those who have fallen, though we should not at all benefit them, still brings us a large reward, hear what Ezekiel says; or rather, what God Himself speaks through him. For when He had sent certain messengers to overturn the city, and to consume all the dwellings with sword and fire, along with their inhabitants, He thus charges one of them: "Set a mark upon the forehead of the men that groan, and are in anguish." And after charging the others, and saying, "Begin ye from mine holy ones," He goes on to add, "But upon whomsoever the sign is, touch them not." For what reason, tell me? Because although they avail nothing, they nevertheless lament the things which are done, and deplore them.