I have never believed Obama was a Muslim.
I also never believed the had any interest in Christianity other than as a pander to voters for political purposes. I don’t think he has gone to church since he was inaugurated.
His mom and grandparents who raised him were atheists. If he were not in politics, I expect he would admit he’s an atheist.
I think the following are also essentially non-believers whose only interest in Christianity is to get votes or avoid losing them:
George W. Bush
I don’t know enough about the others. Romney, Carson, Huckabee, Santorum, and some others appear to be Christians. But in general, these guys famous for smoking pot in college, who stop going to church the day after inauguration, who behave in unChristian ways, are not credible Christians to me.
Furthermore, I don’t believe most Americans are Christians anymore. I know they say they are. But actions speak louder than words. Make anyone who claims to be a Christian account for the last 50 Sunday Mornings. As far as I know, no one I know goes to church. Some of them may tell me I am mistaken, that they just don’t mention it. But it’s hard to never mention it over a long period if they are regular church goers.
My mom claimed to be a Catholic. She went to Church. The Catholic clergy are sort of Nazis about demanding you go to church. Damned to hell forever is the punishment for missing mass once. The Catholic business model is passing the plate in church, or was when I went to Catholic school.
But I never saw or heard my mom say anything that indicated she feared God or the Pearly Gates or looking forward to hanging around with old friends and relatives agains in heaven. Except for going to mass, she never did any Catholic stuff. It seemed to be some sort of family and friends social club and habit or tradition. My mom and most Americans would be horrified to be accused of being atheists—but only because of the ostracism implications, not theology. I think those same people would be equally horrified at having to defend their faith in a logical debate with, say Richard Dawkins.