Mark Thau. Thau, or Tau, is the last letter in the Hebrew alphabet, and signifies a sign or a mark: which is the reason why some translators render this place set a mark, or mark a mark, without specifying what this mark was. But St. Jerome, and other interpreters, conclude it was the form of the letter thau, which, in the ancient Hebrew character, was the form of a cross. (Challoner)

Of this many inscriptions still extant bear witness. (Montfaucon.)

Some Rabbins allow that the last letter was used but in honour of “the law,” Thorah. The cross is supposed to be the hieroglyphic of a future life, (Hist. Rufini. ii. 29.) and found frequently in the pictures or (Haydock) in the tables of Isis. But it rather represents a key. Soldiers who were acquitted received the letter T, and those who were sentenced to die had Th, (Calmet) alluding to Thanatos, “death.” (Haydock)

We may, however, suppose that if God designated any letter, it would be some letter of the Hebrew alphabet, and accordingly the last had formerly the figure of †. x. though this text may signify “a sign” in general. The virtuous would be discriminated from the guilty, as if they were marked. (Calmet)

The door-posts of the Hebrews were stained with blood, in Egypt, to shew that all should be redeemed by that of Christ; and here those who shall be saved, received the mark of his cross. This sign has always been held in veneration among Christians, (Worthington) and used in conferring baptism, consecrating the blessed Eucharist, &c. (St. Chrysostom, hom. lv. in Matthew, and lxxxiv. in John) (St. Augustine, tr. cxviii. in John, and ser. ci. de temp., &c.)

It appeared to Constantine with this inscription, “In this conquer;” (Eusebius, vit. i. 22.) and again over Jerusalem; (St. Cyril, ep. ad Constantium.) and will be borne before Christ, at his last coming, (Matthew xxiv.) to the joy of those who have performed their baptismal promises, and to the confusion (Worthington) of the enemies of the cross of Christ. (Haydock)