Thinking you need religion and otherworldly deities to learn to have morality is like a little kid thinking he needs the reward of presents or the threat of a whuppin' from his daddy for him to behave. It's OK for a little kid. It's unseemly for an adult. Who would you trust more with their moral judgment, a little kid who cleans up his room, doesn't pull his little sister's hair, and takes baths regularly just because of the promise of presents or a whuppin' from his parents, or a mature young man who knows why he should learn to behave for its own sake?
The founders of this country knew that for important ethical/moral/social policy positions, you had to use reason, consultation, negotiation, and compromise. The issues had to be decided on their own merits. SOmeone who is behaving only because otherworldly deities exist and he is afraid of what they will do to him if he doesn't lacks the maturity and judgment to be trusted with any important decisions requiring careful deliberation, judgment, critical thinking, and ability to negotiate and compromise with others. They are just a sheep who will do anything as long as you tack on a "God says..." at the beginning of it. For them, morality is nothing more than a childish game of "Simon says". That may work for the large mass of sheep in the population who can't think for themselves. But for those in positions of power and decision making, that is not acceptable. They need to be able to think for themselves.
You don't need otherworldly deities to have virtue.
But it does me no injury for my neighbor to say there are twenty gods or no God. It neither picks my pocket nor breaks my leg."
-Thomas Jefferson, Notes on Virginia, 1782
The problem with the mass of sheep who always need a "God says" in front of any moral position to learn to behave, they are too easily exploited and manipulated by those wily politicians, priests, and clergy to advance their own power and interests. That is why at the highest levels, this kind of thinking should not be tolerated. That is the rationale behind the separation of church and state.
During almost fifteen centuries has the legal establishment of Christianity been on trial. What have been its fruits? More or less in all places, pride and indolence in the Clergy, ignorance and servility in the laity, in both, superstition, bigotry and persecution...In some instances they have been seen to erect a spiritual tyranny on the ruins of the Civil authority; in many instances they have been seen upholding the thrones of political tyranny: in no instance have they been seen the guardians of the liberties of the people. Rulers who wished to subvert the public liberty, may have found an established Clergy convenient auxiliaries. A just Government instituted to secure & perpetuate it needs them not."
-James Madison
If people want to remain sheep in this country, let them do it on their own time. They are easily exploited and governed.
"The purpose of separation of church and state is to keep forever from these shores the ceaseless strife that has soaked the soil of Europe in blood for centuries."
-James Madison
But public affairs are too important to let them into positions of actual important decision making, and the possibility of things quickly breaking down into bigotry, tribalism, and bloodshed are too high when everyone's argument for any position is "because God said so", and they have no other argument. That is why these founders were OK promoting religion for the masses. They were all politicians still, after all. But in private, their letters were a very different story, as you can tell.
"Because God said so" is not an argument that should be tolerated in any open system of democracy. Anyone can tack on any opinion to the end of that phrase. The founding fathers decided people needed to be able to do a little more than that.
