Ezekiel 9:4

Command to seal those to be spared, and to slay without distinction all others

4. set a mark The word is Tav, the last letter of the alphabet, the old form of which was a cross. The term is used here as in Job 31:35, of a mark in general, though perhaps the Tav or cross was the simplest form the mark could take. The passage is imitated, Revelation 7, though the mark there is the name of God. All who mourned over the abominations done in Jerusalem were to be thus sealed and spared.