Finally, I don't see that the 1st Amendment forever protects tax exemptions
Interestingly, the First Amendment does not protect tax exemption at all. Congress or any state legislature (at least in any state where the state constitution does not prohibit it) could enact new tax laws tomorrow taxing religion and it would without question be constitutional. It would just be political suicide, so it's not happening in the U.S. anytime soon. Taxing some religions and not others based on the content of the beliefs could present a problem however.