Symbolism, Imagery, Allegory
It's not hard to pick out the obvious symbolism here: Christianity.
But the crucifix symbolizes more than just that in this novel. It also represents the Afterlife, society's acceptance of it, and man's collective search for a higher order or meaning that renders significant an otherwise absurd life.
In stark contrast to the Absurdist that is Meursault (and Camus), the crucifix also represents everything that Meursault does not believe in. Rejecting it twice (once from the magistrate judge and the second time from the chaplain), Meursault detests the notion that his life must have any rational explanation or significance. Meursault defies what he sees as a futile attempt at the search for meaning, or for something larger and grander. He doesn't believe in it. Period.