That said, at this point the Supreme Court has essentially made the Separation of Church and State a thing. Basically, the government is not allowed to promote any one religion over others (there's a lot of other requirements too, as it's a very very fine line, but that's the gist of it). From a legal standpoint, the Courts usually try to keep the two separate. But a lot of people these days still believe that Church and State should be unified. They point to the history of the United States frequently, claiming that the Courts have misinterpreted the founding fathers' intentions. For example, in the Declaration of Independence, (which really doesn't hold a lot of legal weight, but is useful for trying to interpret the founding fathers' intentions) they refer to a "Creator" and "Nature's God". One of the most famous written lines in history reads:
The Declaration of Independence wrote:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
The phrase "endowed by their Creator" is pretty obviously religious in nature, though the phrase never appears in the Constitution.
Basically, the whole Separation of Church and State is a relatively modern occurrence, and there's a lot of people (notably the Right) that feel the founding fathers' intentions were not to separate the government and religion so. These same people often feel that a God-fearing, Christian nation is what will set us on the path to happiness, and these are often the people that wouldn't vote for an Atheist president.
Oh my God I'm becoming gbaji.