The anaphoric article, the indefinite article, and the partitive genitive.

Recall the grammatically awkward sentence demonstrating the need for a definite (anaphoric) article. "I came upon a stream in the wilderness.  A stream was badly polluted."  One might object that the sentence was the product of reverse engineering designed to create a grammatical absurdity, but that the use of the indefinite article in the phrase "an establishing" [the participial rendition of "an establishment"] is simply not equally awkward.  There is a kernel of truth to this objection.  When something is divisible into individual components, the indefinite article may become proper despite an antecedent.

The word "stream" is a single entity, but has multiple sub-components%u2014fish, shores, rocks, water.  Addressed as a collective, one of these sub-components still requires a definite article.  "The fish were jumping out of the water,"  or "the shores were polluted."  Even though no fish or shores were previously mentioned, the stream itself has been mentioned already, and the anaphoric use of the definite article properly relates the fish or the shores to the aforementioned stream.

But what if a single fish jumped out?  "One of the fish jumped out of the water."  The word "of" establishes the genitive case.  When the genitive it is used to indicate "a part of the whole," (as here), it is known as a partitive genitive.  We still see the definite article used along with the partitive genitive.  "One of the fish jumped out of the water."  However, an indefinite article could be substituted for the partitive genitive.  "A fish jumped out of the water."  In this way, even when an entity refers back to something already addressed, an anaphoric article may not be necessary, or even proper.  The indefinite article becomes proper when distinguishing a single entity as a "part of the whole."

Obviously there exists within Congress the potential for passing a multiplicity of laws establishing a religion.  It could be argued that this potential multiplicity more closely resembles a multiplicity of fish (where an indefinite article was allowed) than the second time the word "stream" was used, which, as a single indivisible entity, required the definite article.  There are two problems with this reasoning.  1.  Logically, one would have to advance some theory for why Congress would want to distinguish between various laws establishing a religion.  A single fish could jump from the stream while the rest remained water-bound.  It is thus distinguished from the other fish.  But it would be foolish to contend that Congress intended to prohibit some laws "establishing" a religion while condoning other such laws "establishing" a religion.   2.  Grammatically speaking, a stream involves a plurality of entities.  It has fish, water, shores, and rocks.  Moreover, each of these entities, fish, shores, and rocks are plural entities.  The indefinite article was sensible when it replaced a partitive genitive, (the word "of" referring to a part of the whole), and the partitive genitive is sensible when referring to a part of a plurality ("one of the shores"), or part of a divisible entity ("a slice of the cake").

It is perhaps possible that the action of establishing a religion is one of many thousands of laws that congress might potentially make.  "Congress shall make no laws (plural) respecting an 'establishing' (participial nuance) of religion."  If the word "law" had been plural, this would be tenable.  But it is singular. . . "law."  An act of 'establishing' a religion is not part of making an individual law, it is equivalent to making a law.  Accordingly, the indefinite article seems inappropriate when the participial nuance is adopted.

The fact that one could equally render it in the plural "Congress shall not make any laws" is irrelevant.  The drafters did not render it in the plural, so grammatical options which attach to the use of the plural are simply not available in analyzing the first amendment.  The framers rendered both words, "law" and "establishment" in the singular.

Conclusion