At this point about the only non-controversial word to use is "Christian" (some extremists would even call that controversial). We can't even come to an easy agreement about what branch of Christianity should be included in the infobox. Adding footnotes about Muslim birth or atheism would spark another huge debate on this page. And frankly, I would be among the debaters. Even a footnoted comment like that puts too too much weight on those issue for this article. There's another article on Obama controversies.