Tau is the last letter of the Hebrew alphabet and was used symbolically in the Old Testament. It was already spoken of in the Book of Ezekiel: “The Lord said to him, ‘Go through the city, through Jerusalem, and put a mark upon the foreheads of the men who sigh and groan over the abominations… (Ez. 9:4). The Tau is the sign placed upon the foreheads of the poor of Israel, it saves them from extermination.
It was then adopted by the very first Christians, for a twofold reason:
As the last letter of the Hebrew alphabet, it prophesied the Last Day and had the same function as the Greek letter Omega as it appears in the Book of Revelation: “I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give water without price from the fountain of the water of life … I am the Alpha and the Omega, the first and the last, the beginning and the end” (Rev. 21:6;22:13).
The Christians adopted the Tau, because its form reminded them of the Cross on which Christ was immolated for the salvation of the world.