Frank, here are other grave markers that are not crosses used for US servicemen: The cross, star of david, etc. represent that persons faith. So yes the cross is a symbol of religion. If you look at headstones that are your typical gravestone shape the religious persuasion of the soldier is signified by a cross, star of david, etc. Teddy Roosevelt did not swear in on a bible. John Quincy Adams swore in on a law book.