To sign or be marked by the cross originates in the Old Testament. In the Book of Ezekiel, the Babylonians threaten to conquer the city of Jerusalem. This is symbolized in Ezekiel's vision by six armed men approaching the city from the north. While they approached, a man with a writing case came and stood in the temple. God instructs this man to go throughout the city and mark those who have persevered in their faith with the sign of the Tav. At the time, this would look closer to an "X," but its more ancient form was closer to the "T," which looks like a cross.