This is borne out by Tertullian, who intimates 1) that "the devil, by the mysteries of his idols, imitates even the main parts of the divine mysteries. He also baptises his worshippers in water, and makes them believe that this purifies them of their crimes......There Mithra sets his mark on the forehead of his soldiers; he celebrates the oblation of bread; he offers an image of the resurrection, and presents at once the crown and the sword; he limits his chief priest to a single marriage: he even has his virgins and his ascetics (continentes)."
1) Præscr. c. 40; Cp. De Rapt. c. 5; De Corona, c. 15.
The "mark on the forehead" of the initiate, finally, was in all likelihood the cross, the universal symbol of life and immortality, and in particular of the Sun-God. Presumably it was not the gammadion or swastika, the most specific symbol of the Sun, for that appears to have been notably absent from Persian art. 8) That it was one of the normal forms of the "Christian" cross may be inferred from the mode of Tertullian's statement, and from the fact that the tau or cross was inferribly a forehead mark in the Judaic cult set forth in the book of Revelation. 1) We know that the symbol entered into the fire-worship of Persia by way of architecture; 2) and it could not have been absent from the imagery of an eastern Sun-God of the time.
8) Goblet d’Alviella, The Migration of Symbols, Eng. tr. 1894, pp. 80-82.
1) Cp. Zœckler, The Cross of Christ, Eng. tr. 1877, pp. 80-81, 105; Rev. vii, 3; xiv, 1; xxii, 4; Ezek. ix, 4; S. Baring Gould, Curious Myths, 1888, pp. 376-7.
2) Justi. Gesch. der oriental. Völker im Altertum, p. 397.