The United States is not and never has been a "Christian nation." America's Founding Fathers explicitly designed the country on pluralist grounds and encouraged Americans to accept people of all religious backgrounds.
This is why the actions of a junior high school teacher named Linda White of Mississippi are so anti-American. White told her students not to vote for Barack Obama because he's Muslim. She instead told her students to vote for Mitt Romney because he's Christian.
Whether Obama is Muslim or not is beside the point. The president's religious background shouldn't matter in a presidential election -- or any American election for that matter. What matters to me is that he or she upholds the Constitution and the Declaration of Independence and sustains the egalitarian principles put forth by the Founding Fathers.
White, however, has committed the ultimate crime of polluting an American classroom with a myth that could potentially corrupt the youth into thinking that America is only for Christians. To encourage young Americans to vote for candidates based on their religious background paves the way for an American future of exclusivity and intolerance, which, ironically, are two issues the Founding Fathers hoped to erase from the world.
Discriminating against someone because of their religious beliefs is the most fundamentally anti-American thing an American could possibly do. The president could be Hindu, Buddhist, Scientologist or atheist for all I care. What matters in leaders, especially the president, is their character and conduct.