That's entirely up to you. If there's some kind of aesthetic value, some sort of nostalgic affect, then I'd say keep it around, what can it hurt?
But you're the one that has to decide whether or not you want symbols you don't agree with hanging on your walls.
All of that aside, however, I promise that you won't spontaneously combust for keeping it around. There's no Satanic 'cross police' itching to come bursting into your home to take it down and bring you up on charges of apostasy.