So, 18 percent of Americans think Barack Obama is a Muslim, according to a Pew Research Center poll, 34 percent believe he's Christian, and 43 percent say they don't know what his religion is. On the other hand:
To put it all in context however it useful to look at what else a similar percentage of Americans believe. CBS commissioned a poll back 2006 on ghosts and found a full 22% of Americans believe they have seen or felt a ghost while a 48% believe ghosts exist.
A poll by CNN found that 37% of Americans polled believed that aliens have contacted the US government.
“Sixty-four percent of the respondents said that aliens have contacted humans, half said they've abducted humans, and 37 percent said they have contacted the U.S. government.
I bet if we started a rumor that the president is the ghost of a Muslim from another planet, we could do a poll in a couple of weeks finding 15 percent of Americans believe it.