So in a sense, yes, the letter to the Danbury Baptists was (indirectly) adopted by the people to be their Constitution. And this is largely how it’s been interpreted by Supreme Courts for the last two hundred years. E.g., Reynolds vs. United States (1879): Jefferson’s observations “may be accepted almost as an authoritative declaration of the scope and effect of the [First] Amendment.”
The Reynolds Court, in my view, used Jefferson’s letter only to “confirm” what it had already derived from documents (Madison’s Memorial and Remonstrance and the Virginia Religion Bill of 1786) written “before” the First Amendment was written. The Court admitted that Jefferson’s letter was not a primary source of law when it said it “almost” authoratative.
The Reynolds Court wrote a really crappy opinion, in my view. It made the right ruling, from the Jeffersonian view of the Constitution, but its reasoning was incorrect.
The Court failed to point out that the Constitution granted the U. S. Government, a government created with limited authority, any jurisdiction whatsoever over religion. However, it correctly realized that the word “religion” was critical but not defined in the Constitution.
It should have been guided by Blackstone’s Third Rule of Construction which says,
3. AS to the fubject matter, words are always to be underftood as having a regard thereto; for that is always fuppofed to be in the eye of the legiflator, and all his expreffions directed to that end. Thus, when a law of our Edward III. forbids all ecclefiaftical perfons to purchafe provifions at Rome, it might feem to prohibit the buying of grain and other victual; but when we confider that the ftatute was made to reprefs the ufurpations of the papal fee, and that the nominations to vacant benefices by the pope were called provifions, we fhall fee that the reftraint is intended to be laid upon fuch provifions only.
Instead, the court turned to the “history of the times in the midst of which the provision was adopted” and found Madison’s Memorial and Remonstrance and the Virginia Religion Bill of 1786. Both of those documents would qualify as “subject matter” under Blackstone’s Third Rule. However, only the M and R provides the meaning of the word “religion.”
(My view is that the Court should have relied upon the writings on the subject of religious liberty, which actually defined the word “religion”, that were closest in time to the writing of the First Amendment. In late 1788, the Virgina Ratifying Convention recommended an amendment to the Constitution which defined religion as “the duty which we owe to our Creator.” This writing should have been used by the Court.)
The Reynolds Court adopted, or at least signaled its approval of, the meaning James Madison gave to the word “religion” in his M and R. At this point the Court should have substituted Madison’s definition of religion for the word “religion” in the First Amendment and arrived at the following construction:
“Congress shall make no law respecting an establishment of a duty which we owe to our Creator, or prohibiting the free exercise of a duty which we owe to our Creator.”
At his point, the Court would have faced the problem of determining whether a Mormon’s duty to take more than one wife was an exercise of a “duty owed to the Creator.” The Court, in my view at least, should have reasoned that it was not, because taking more than one wife involves more than a man and his God. It involves a Man, his God, his first wife and his second wife.
(A duty to God is one that is owed only to God only and has no effect on other men. A duty that involves other men is what Jefferson termed a “social duty.”)
What the Court actually did was determine that bigamy was not a “free exercise religion” under the First Amendment because, “Polygamy has always been odious among the northern and western nations of Europe, and, until the establishment of the Mormon Church, was almost exclusively a feature of the life of Asiatic and of African people.” The Court ruled correctly, assuming that Madison’s view of the First Amendment is correct, that polygamy was not “religion” under the First Amendment, However, its reasoning was incorrect. The meaning of the Constitution is not to gathered from history. It is to gathered from the words of the Constitution and the well established common law rules of construction.
There are no rules to determine what “historical events” or “historical interpretations” should be used to determine the meaning of the Constitution. All it does is provide a playground for judicial activism. A good example is Justice Rehnquist’s abuse of history to justify his interpretation of the establishment clause to do nothing more than prohibit a national religion.