The political mortality of the founders WAS libertarian. Jefferson was certainly libertarian in his political morality and his religious views would shock many. Thomas Paine, who helped kick off the revolution with his pamphlet Common Sense, wrote an entire book on Bible contradictions. The first amendment seems to suggest general opposition to an official government morality, too.
If we REALLY want to go back to the founders' ideas about morality, we'd go back to state's rights (or community rights, IMO) and let each community suffer or succeed based on the fitness of its morality, letting God, or Nature, or fate, or Physics, or Divine Providence, or whatever word you prefer, decide the outcome.
Unfortunately it doesn't work like that. Condoning immorality by doing nothing to oppose it, is tacit approval. When the immoral nation goes down, all suffer, including the citizens who are moral.
You are confusing the founder's opinions toward particular biblical doctrines with their views on virtue. An additional and vital purpose of religious freedom was so that the common virtues espoused by the world's faith trandition could have easy access to the public square. As Madison said, "before one can be a member of civil society, he must be subject to the Governor of the Universe." The founders recognized that there was a civil manifestation of faith in God. . .not as dogma, but rather as virtues that were germaine to the practice of good citizenship. . .thus the need to adhere to "the laws of nature and of nature's God". .that is, to be subject to the Governor of the Universe, was essential for a people to be free. Thus "Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other." John Adams
"Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other." --October 11, 1798 John Adams