This Tau was in the form of the cross of this degree, and it was the emblem of life and salvation. The Samaritan Tau and the Ethiopic Tavvi are the evident prototype of the Greek r; and we learn from Tertullian, Origen, and St. Jerome, that the Hebrew Tau was anciently written in the form of a Cross.In ancient times, the mark Tau was set on those who had been acquitted by their judges, as a symbol of innocence. The military commanders placed it on soldiers who escaped unhurt from the field of battle, as a sign of their safety under the Divine Protection.It was a sacred symbol among the Druids. Divesting a tree of part of its branches, they left it in the shape of a Tau Cross, preserved it carefully, and consecrated it with solemn ceremonies. On the tree they cut deeply the word Thau, by which they meant God. On the right arm of the Cross, they inscribed the word Hesuls, on the left Belen or Belenus, and on the middle of the trunk Tharamis. This represented the sacred Triad.
The TAU cross was inscribed on the forehead of every person admitted into the Mysteries of Mithras. When a king was initiated into the Egyptian Mysteries, the TAU was placed against his lips. It was tattooed upon the bodies of the candidates in some of the American Indian Mysteries. To the Qabbalist, the TAU stood for heaven and the Pythagorean tetractys. The Caduceus of Hermes was an outgrowth of the TAU cross. (See Albert Pike.)
Source:
http://gnosticwarrior.com/the-true-cross.html
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