and what is its significance in christianity?
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what was ss christopher columbus? An American excursion liner on the Great Lakes where is this lake situated? The upper mid-east region of North America which religions are practiced in that continent? Christianity, Atheism,  Buddhism what are the symbols associated with the first religion? The cross are there emblems like that symbol? Yes what was its significance in pre christian era? In the European Bronze Age the cross symbol appeared to carry a religious meaning, perhaps as a symbol of consecration, especially pertaining to burial.
It is seen as a representation of the instrument of the crucifixion of Jesus, is the best-known symbol of Christianity. The shape of the cross, as represented by the letter T, came to be used as a "seal" or symbol of Early Christianity by the 2nd century. A tradition for Christians to trace repeatedly on their foreheads the sign of the cross.
Christian cross Cross Cross
Introduction Christian cross Christian cross
 seen as a representation of the instrument of the crucifixion of Jesus, is the best-known symbol of Christianity. The shape of the cross ("crux", "stauros" "stake, gibbet"), as represented by the letter T, came to be used as a "seal" or symbol of Early Christianity by the 2nd century.  The shape of the cross ("crux", "stauros" "stake, gibbet"), as represented by the letter T, came to be used as a "seal" or symbol of Early Christianity by the 2nd century. Clement of Alexandria in the early 3rd century calls it ("the Lord's sign") he repeats the idea, current as early as the Epistle of Barnabas, that the number 318 (in Greek numerals, ΤΙΗ) in Genesis 14:14 was a foreshadowing (a "type") of the cross (the letter Tau) and of Jesus (the letters Iota Eta). Clement's contemporary Tertullian rejects the accusation that Christians are "crucis religiosi" (i.e. "adorers of the gibbet"), and returns the accusation by likening the worship of pagan idols to the worship of poles or stakes. 
In his book "De Corona", written in 204, Tertullian tells how it was already a tradition for Christians to trace repeatedly on their foreheads the sign of the cross.
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Cross [SEP] Pre-Christian
Due to the simplicity of the design (two intersecting lines), cross-shaped incisions make their appearance from deep prehistory; as petroglyphs in European cult caves, dating back to the beginning of the Upper Paleolithic, and throughout prehistory to the Iron Age. Also of prehistoric age are numerous variants of the simple cross mark, including the "crux gammata" with curving or angular lines, and the Egyptian "crux ansata" with a loop. Speculation has associated the cross symbol – even in the prehistoric period – with astronomical or cosmological symbology involving "four elements" (Chevalier, 1997) or the cardinal points, or the unity of a vertical axis mundi or celestial pole with the horizontal world (Koch, 1955).