Well, if the U.S.Constitution is written in such a way that states can have official state religions, how long do you think it will be before there is an amendment ginned up by Congress to seal that off.
Having a bruising fight in Congress is much preferable to having an openly fought civil war, IMHO.
Uhhhhhhh, we're at 230 years and counting. Congress hasn't tried to amend the First Amendment yet, but they've had a long time to consider doing so. So far: no interest.