U.S. Army infidel patches and apparel?

I have seen various army personnel wearing patches on their acu's that say infidel, I have also seen through my own research various companys selling shirts stickers and other things that say us army infidel. what is the meaning of this, is it unit specific and as a civi about to enlist at meps as a 19D is this something that i would be able to wear myself?