The Mark of Preservation

Ezekiel 9:3-4

And the glory of the Lord of Israel went up from the cherub, upon which he was, to the threshold of the house: and he called to the man that was clothed with linen, and had a writer's inkhorn at his loins.

And the Lord said to him: Go through the midst of the city, through the midst of Jerusalem: and mark Thau upon the foreheads of the men that sigh, and mourn for all the abominations that are committed in the midst thereof.