Some countries have established religions or state churches; I assume we're talking about the United States, which doesn't. I don't know whether federal and various state laws / regulations / case law require belief in spooks and magic for those tax breaks and other privileges.
How are Ethical Culture societies treated?
(I do vaguely remember some state marriage laws which allowed solemnization by a judge or by a "minister of the gospel".)
Just some links The Freedom from Religion Foundation and the IRS have put out good summaries of information on this issue. It appears, from the IRS perspective, that it was the Tax Reform Act which really cut the church a break.
The line separating church and state is becoming fuzzier and fuzzier. We do not want to make the situation worse.
OK.. just because the RCC rips off the government we can't