2. This is not true to begin with.
Religious affiliations of United States Presidents - Wikipedia, the free encyclopedia
Thomas Jefferson, Franklin Pierce, Abraham Lincoln, Andrew Johnson, and Rutherford B Hayes had no affiliation to any church when they were elected President (not counting Eisenhower, who was baptized 12 days into office, or Obama, who broke off ties with his church during the campaign).
Of those, at least Jefferson was not a Christian, and possibly Lincoln as well.
So 11% did not attend church, and 2-4.5% were not Christian.
I think the main reason why it would be difficult to get an atheist elected President is because many people associate atheism with ferocious hatred and anger at people for believing something that they do not personally believe, and then saying that those people are oppressing atheists by making their religion "political".