Eugene Volokh has an interesting post which references law review articles from among others Akhil Amar on the idea of "intratextualism" -- that is, the same word being used more than one time in a legal document and how the multiple uses of a term can define its proper meaning.
Here is a comment I left:
I’ve studied the “religion” at the time of the framing in detail from the perspective of “political theology,” and the issue certainly applies.
The First Amendment says
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;
And Art. VI Cl. 3 of the unamended Constitution says
no religious test shall ever be required as a qualification to any office or public trust under the United States.
So we’ve got “religion” and “religious” as in “religious test.” It probably would be proper to define "religion" as having the same meaning in both clauses, but as a matter of logic we MUST define “religion” in the EC and FEC as having the same meaning because the term is used only once and “thereof” in the FEC relates back to “religion” in the EC. It’s like a Siamese twin that shares the same heart.
The reason I say this is, perhaps because they misunderstand a quotation of Joseph Story, some Christian Nationalists have argued either 1. “Religion” in the First Amendment meant “Christianity” only (this claim is wrong but at least it avoids the problem of logical construction where “religion” in the EC and FEC are given two different meanings) OR 2. Whereas Free Exercise was meant to apply universally the EC somehow only protected or privileged “Christianity.” This claim is wrong as a matter of simple logic. If one can prove “religion” had a particular free exercise clause meaning, it automatically applies to the establishment clause and vice versa. (And yes, there is a great deal of evidence that the Founders meant what they said: “religion” means “religion” not “Christianity”).