Set a mark— This expression alludes to the ancient custom of marking servants in the forehead, to distinguish what they were, and to whom they belonged. See Bishop Newton on Revelation 7:3. The reader is to remember, that all this passed in vision, and only means that God made a distinction, and separated the good from the bad, as really as if he had marked them with some visible sign. This parabolic command, says Bishop Warburton, alludes to the sanction of the Mosaic law; and implies, that virtuous individuals should be distinguished from the wicked in a general calamity.
Related Commentaries of Ezekiel 9:4
Ezekiel 9:4
4 And the LORD said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a markb upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof.