“The fears of the Christians were but too well founded, and their pious zeal and prophetic insight was rewarded from the very first. In the demolition of the Serapeum, after the bloody riot between the Christian mob and the Pagan worshippers had ended with the interference of the emperor, a Latin cross, of a perfect Christian shape, was discovered hewn upon the granite slabs of the adytum. This was a lucky discovery, indeed; and the monks did not fail to claim that the cross had been hallowed by the Pagans in a “spirit of prophecy.” At least, Sozomen, with an air of triumph, records the fact. But, archeology and symbolism, those tireless and implacable enemies of clerical false pretenses, have found in the hieroglyphics of the legend running around the design, at least a partial interpretation of its meaning.
According to King and other numismatists and archeologists, the cross was placed there as the symbol of eternal life. Such a Tau, or Egyptian cross, was used in the Bacchic and Eleusinian Mysteries. Symbol of the dual generative power, it was laid upon the breast of the initiate, after his “new birth” was accomplished, and the Mystae had returned from their baptism in the sea. It was a mystic sign that his spiritual birth had regenerated and united his astral soul with his divine spirit, and that he was ready to ascend in spirit to the blessed abodes of light and glory – the Eleusinia.
The Tau was a magic talisman at the same time as a religious emblem. It was adopted by the Christians through the Gnostics and kabalists, who used it largely, as their numerous gems testify, and who had the Tau, (or handled cross), from the Egyptians, and the Latin cross from the Buddhist missionaries, who brought it from India, where it can be found until now, two or three centuries B.C.
The Assyrians, Egyptians, ancient Americans, Hindus, and Romans had it in various, but very slight modifications of shape. Till very late in the mediaeval ages, it was considered a potent spell against epilepsy and demoniacal possession; and the “signet of the living God”, brought down in St. John’s vision by the angel ascending from the east to “seal the servants of our God in their foreheads”, was but the same Mystic Tau – the Egyptian cross.
In the painted glass of St. Dionysus, (France), this angel is represented as stamping this sign on the forehead of the elect; the legend reads, SIGNVM TAY. In King’s Gnostics, the author reminds us that “this mark is commonly borne by St. Anthony, an Egyptian recluse.” What the real meaning of the Tau was, is explained to us by the Christian St. John, the Egyptian Hermes, and the Hindu Brahmans. It is but too evident that, with the apostle, at least, it meant the “Ineffable Name”, as he calls this “signet of the living God”, a few chapters further on, the “Father’s name written in their foreheads.””
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 