Scott Walker isn't sure if President Obama is Christian. Many other conservatives are quite sure that he isn't. Many liberals are equally certain that the Islamic State isn't Muslim. Two problems. First, inasmuch as religion is about belonging, this is all quite simply about delineating group boundaries. See the No True Scotsman Fallacy. Second, it's all fruitless anyway since it rests on the assumptions that "Christianity" and "Islam" are monolithic institutions. In reality, there are many Christianities and many Islams. (See previous posts here and here.) Pretending otherwise is only about maintaining oneself as the legitimate arbiter in problem one above. Rinse and repeat.
Darren Sherkat has a good related post--albeit in his typical style.