It can not be mistaken that two lines perpendicular to each other where one line stands to divide the other line in half is a cross. The cross is almost always tied to the idea of faith, belief, divinity and, of course, Christianity. Since time in memoriam, crosses are used. While it may not be largely religious, its religious innuendo is never separated from it.
People who wear a cross pendant in their necklaces probably do it as allusion to its symbolism: this is faith. I have faith.
The significance of the cross is largely of Christian origin. It serves as a reminder of the crucifixion of Jesus Christ as a means of salvation. According to Christian belief, Jesus was crucified and died on the cross to save humanity from their sins.
While being largely Christian, the cross also serve to stand for other non-Christian things. The cross is seen as a symbol for life in Egyptian culture—or specifically in the symbol of the ankh.
Whatever declaration one has for wearing the cross: for a declaration of Christianity, for a declaration of faith, divinity or the love for life, wearing a cross pendant is great way to do it.