How can a clearly written, 16-word sentence be so blatantly misinterpreted?
A and an are indefinite articles that always precede nouns and can be replaced by the word any. Establishment is the noun in this case, a religious establishment or any religious establishment. The clincher is the word thereof in the dependent clause that follows. Thereof must be able to be replaced by the object of the first clause and still make sense.
Congress shall make no law respecting any religious establishment, or prohibiting the free exercise of any religious establishment. Makes sense.
Congress shall make no law respecting a state established religion, or prohibiting the free exercise of a state established religion. Makes no sense.
There is no establishment clause. Congress can't fund any religion.