I find that the question of their own religious beliefs is highly irrelevant to the discussion. What they wrote is what matters, because that is what they all agreed was the best foundation for the country. Likewise, I find that the states who declared themselves Christian (or, more accurately, a specific flavor of protestant) really have no bearing on whether America was founded as a "Christian nation" or a "secular nation," they are merely a subset of the whole, and the power of state's rights has differed greatly over time. The foundational document of the country was express in its intent to let people worship in peace as they pleased. Perhaps they could have never forseen the influx of different, non-Christian religions, or atheism, but that's ok because it was never written by then that that mattered.

Again, I would highlight that the founders were deeply influenced by Enlightenment thinkers, many of whom fit in the atheistic-agnostic-deistic spectrum, and who were express in their philosophies that governments should be secular. In fact, I consider Thomas Paine to be a founding father and he was explicitly not Christian. They were very worldly folks and knew philosophy and political science, and understood religion's place was the individual and their community, not the government. Individual states may have differed but the federal government of the United States was never, ever "Christian."