I don’t think it is “most” but I’d be willing to stipulate that is “a lot.”
http://www.gallup.com/poll/155315/many-americans-cant-name-obamas-religion.aspx
Gallup thinks it is most: “Just 34% of Americans correctly say U.S. President Barack Obama is a Christian, while 44% say they don’t know Obama’s religion and 11% say he is a Muslim.”