Most churches raise money by selling the postcards and other paraphernalia....and, as has been said above, churches can be private property, owned by the church bodies or local councils
In the UK, parish churches are not owned by anybody, but the churchwarden owns the contents. parishioners have the rights only to burial there and to worship inside
Therefore, it appears that you could take photos of the exterior but, all inside belongings belong to the warden, so he has a right to say no to you selling photographs, or even taking them.
This may well be the case for some of the churches in the US like the Anglican ones etc but others like JW halls etc are owned by the JW's