Amazingly, 18% of Americans apparently believe that President Obama is a Muslim. So reports the Pew Research Center's poll done in late July-early August. That's up from 11% in March 2009 - and before the current controversy erupted about the Mosque near Ground Zero. According to an even more recent Time magazine poll, 24% believe the President is a Muslim, 47% believe he is a Christian, and 24% say they don't know!
Now one of the features of contemporary life is an ideological disdain for facts. Perhaps the most blatant example of this is the absurd belief, which some people insist on maintaining without any evidence, that the President is not a natural-born American citizen. In the current climate, people on either extreme of the political-cultural divide increasingly get their information exclusively from fellow true-believers and accept as facts only whatever confirms their pre-existing ideological stance - however counter-factual such supposed "facts" may be. (This is not a totally new phenomenon. Remember all those lefties who refused to believe that Oswald alone killed JFK. But a way of thinking that used to be recognized as extreme and marginal has become much more common and mainstream).
So maybe one shouldn't be so surprised that opponents of the President choose to believe that he is a Muslim. Much more significant, however, is the surprising fact that almost a quarter of the population may say that they do not know what the President's religion is!
To the perpetual dismay of our secularist elite, the United Sates remains the Western world's most religious country. Americans seem to prefer that their presidents practice some religion. Like their fellow citizens, our presidents have been all over the map of religious belief and observance - from fervent Evangelical (Jimmy Carter, George W. Bush) to virtual Deist (Thomas Jefferson). With one Roman Catholic exception, however, all our presidents have been at least nominally Protestant, and most of them seem to have acted more or less according to expectations. Although only baptized after his election, President Dwight D. Eisenhower attended church regularly and even invited Soviet ruler Nikita Kruschev to accompany him to church during the latter's 1959 state visit. (He didn't.) Among recent presidents prior to Obama, Ronald Reagan was conspicuous for not being a regular churchgoer. He got away with it, I suspect, because his claim to be a Christian was nonetheless seen as sincere (and perhaps not unlike that of many other non-churchgoing Americans), and also because Reagan so successfully identified himself with core American values.
Despite his upbringing, President Obama definitely has identified as a Christian. In Chicago, he attended church and was affiliated with the United Church of Christ denomination. Since becoming president, however, he has not affiliated with any church. Nor does he employ Christian religious rhetoric as frequently and fervently as many of his predecessors (including Reagan). The latter may be even more relevant than the lack of church attendance. Under different circumstances, that might not matter for there to be a widespread popular perception of Obama as a Christian. In the present situation, however, it apparently does.