It doesn't matter what the Constitution has to say on the matter, either. All the First Amendment says is that the government is prohibited from setting up an "official American Church" or from stopping anyone from worshiping however they see fit.
But don't get scared. It doesn't mean you have to shut up about your religion just because it annoys some people. It doesn't mean you have to stop practicing your religion.
What freedom of/from religion does mean, is that if you are in a position where you have power over someone else, rightly or wrongly, and that power is due to your government "job", you need to keep your religion in your pants, and keep your pants zipped. Until you are "off the clock", anyway. You have no authority to be preaching at people you are trying to coerce while they are paying you with money you had stolen from them on your behalf.
It means that what you believe on your own time is your business, but trying to pass or enforce "laws" based upon your religious beliefs- Sharia Law- is outside of what you have a right to do. It means using stolen money to promote your religious beliefs- any religious beliefs- is wrong.
Freedom of religion equals, in many cases, freedom from religion. Whether you like it or not. And believe me, even those who claim otherwise usually demand to be free of having other religions' ideas forced upon them- they just don't want to stop doing the forcing themselves. At least, that's what I have observed.