In the context of the Government subscribing to a particular religion, I use the word “religion”, but I guess I really mean “religious belief”, i.e. a belief about religion, in the broadest sense. I would consider deism to be a religious belief under that definition.

So to reiterate my point, if you, the designer of a system of government, allow the state to hold and enforce a religious belief of any kind, eventually a government will take power which holds a different religious belief, and use the state"s ability to deal with religious matters to enforce their different belief upon the people. And this will inevitably happen. So the best protection you can design against this is to withhold this power from the state by explicitly declaring it to be secular.