In today's first reading of the Holy Mass, Ezekiel (9:4) describes how the faithful are marked with a "tau" on the forehead to preserve them from destruction. The "tau" is the letter "t" and according to the Fathers it corresponds to the sign of the cross received on the forehead at baptism.
In his commentary on Ezekiel, Origen interprets the meaning of the mark:
“The shape of the letter ‘Tau’ presented a resemblance to the figure of the Cross and that therein was contained a prophesy of the sign which is made by Christians upon their foreheads, for all the faithful make this sign in commencing any undertaking and especially at the beginning of prayer or of reading Holy Scripture. (Origen, In Ezekiel, 3)
This marking on the forehead is mocked by Satan with the Apocalyptic "mark of the Beast". Click here to listen to the podcast on "Mark of the Beast and Caesar" by Taylor Marshall.