What is the bumper sticker "God isn’t big enough for one religion" implying?

Notably plastered among other stickers such as “Proud parent of a soldier”, etc…

I’ve seen it on a few cars. But I’m having trouble…figuring out what it’s trying to imply. I’m versed in the teachings of Christianity, although Agnostic, so I’m not completely stupid when it comes to this sort of thing. I can’t seem to put my finger on what this statement is trying to get across.