“Make a plate of pure gold and engrave on it as on a seal: holy to the Lord. Fasten a blue cord to it to attach it to the turban; it is to be on the front of the turban.” (Exodus 28:36-37)
Literal Sense: In the Old Covenant, the High Priest wore a golden plate on his forehead which read “holy to the Lord” in the ancient tongue of the Hebrews.
Allegorical Sense: In the New Covenant, it is not only the High Priest who is marked out as holy to the Lord but all of God’s people are marked on the forehead with the sign of the cross, as the Scriptures say: “[d]o not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God.” (Revelation 7:3)
Moral Sense: If cattle, which serve their owners, are marked with a sign in order to indicate ownership, how much more are we who have been marked with the sign of the cross obligated to serve our Master?
Anagogical Sense: Now that we have been marked with the sign of the cross, if we persevere in the faith we will be holy to the Lord for all of eternity.