Smithsonian has a fine article on the real history behind America's status as a "Christian nation": it just isn't so. Religion is a poison our European ancestors brought to these shores, and it's been a source of trouble and stupidity since the beginning.
From the earliest arrival of Europeans on America's shores, religion has often been a cudgel, used to discriminate, suppress and even kill the foreign, the "heretic" and the "unbeliever"—including the "heathen" natives already here. Moreover, while it is true that the vast majority of early-generation Americans were Christian, the pitched battles between various Protestant sects and, more explosively, between Protestants and Catholics, present an unavoidable contradiction to the widely held notion that America is a "Christian nation."
We are a nation of diverse and competing faiths. And we've been made weaker because of it.