In 1896 a man by the name of John Denham Parsons wrote a book on the history of the cross. Since he had been a young man several questions regarding the cross had puzzled him and when he was older he determined to try and and answer them for himself.
The first of those questions was why John the Baptist, who was beheaded before Jesus was executed, and so far as we are told never had anything to do with a cross, is represented in religious pictures as holding a cross.
The second question was whether this curious but perhaps in itself easily explained practice had in its inception any connection with the non-Mosaic initiatory rite of baptism; which Jesus accepted as a matter of course at the hands of his cousin John, and in which the sign of the cross has for ages been the all-important feature. And it was the wonder whether there was or was not some association between the facts that the New Testament writers give no explanation whatever of the origin of baptism as an initiatory rite, that this non-Mosaic initiatory rite was in use among Sun-God worshippers long before our era, and that the Fathers admitted that the followers of the Persian conception of the Sun-God marked their initiates upon the forehead like the followers of the Christ, which finally induced the author to start a systematic enquiry into the history of the cross as a symbol.
The third question was why, despite the fact that the instrument of execution to which Jesus was affixed can have had but one shape, almost any kind of cross is accepted as a symbol of faith.
The last of the four questions was why many varieties of the cross of four equal arms, which certainly was not a representation of an instrument of execution, were accepted by Christians as symbols of the Christ before any cross which could possibly have been a representation of an instrument of execution was given a place among the symbols of Christianity; while even nowadays one variety of the cross of four equal arms is the favourite symbol of the Greek Church, and both it and the other varieties enter into the ornamentation of our sacred properties and dispute the supremacy with the cross which has one of its arms longer than the other three.
Pursuing these matters for himself, he eventually found that even before the christian era the cross was venerated by many as the symbol of Life; though works of reference seldom mention this fact, and never do it justice.
He moreover discovered that no one had ever written a complete history of the symbol, showing the possibility that the stauros or post to which Jesus was affixed was not cross-shaped, and the certainty that, in any case, what eventually became the symbol of christian faith owed some of its prestige as a Christian symbol of Victory and Life to the position it occupied in pre-Christian days.
is the full transcript of the book THE NON-CHRISTIAN CROSS: AN ENQUIRY INTO IT'S ORIGIN AND HISTORY by John Denham Parsons
This is a fascinating book for anyone interested in the early history of the church and the origins of its symbols, particularly the cross (with some surprising answers) and is written in very accessible language and I can thoroughly recommend it.