The short answer? Ever since the beginning. Thomas Jefferson had to explain to the Calvinists in Connecticut how the establishment clause worked because it only took them a few years after the constitution was written to start trying to impress denomination specific rules into law.

There’s been people warning about this since forever. Hell there’s literally a novel written by the late Robert Heinlein called “if this goes on-” wherein the USA has been destroyed and replaced by the kind of society modern conservative religious authoritarians have wet dreams about. He wrote in a post-word at the end of the novel how he was noticing religious extremism taking over the Republican party, and this was in the early 50’s.

“As for the second notion, that we could lose our freedom by succumbing to a wave of religious hysteria, I am not sorry to say that I consider it possible. I hope that it is not probable. But there is a latent deep strain of religious fanaticism in this, our culture; it is rooted in our history and it has broken out many times in the past. It is with us now; there has been a sharp rise in strongly evangelical sects in this country in recent years, some of which hold beliefs theocratic in the extreme, anti-intellectual, anti-scientific, and anti-libertarian (Note: his intended meaning is the political compass version of libertarian, not the modern right wing group.)

It is a truism that almost any sect, cult, or religion will legislate it’s creed into law if it acquires the political power to do so, and will follow it by suppressing opposition, subverting all education to seize early the minds of the young, and by killing, locking up, or driving underground all heretics. This is equally true whether the faith is communism or holy-rollerism; indeed it is the bounden duty of the faithful to do so. The custodians of the true faith cannot logically admit tolerance of heresy to be a virtue.

Nevertheless this business if legislating religious beliefs into law has never been more than sporadically successful in this country- Sunday closing laws here and there, birth control legislation in spots, the prohibition experiment, temporary enclaves of theocracy like Voliva’s Zion, Smith’s Nauvoo, a few others. The country is split up into such a variety of faiths and sects that a degree of uneasy tolerance now exists from expediant compromise; the minorities constitute a majority of opposition against each other.

Could it be otherwise here? Could any one sect obtain a majority at the polls and take over the country? Perhaps not- but the combination of a dynamic evangelist, television, enough money, and modern techniques in advertising and propaganda might make Billy Sunday’s efforts look like a corner store compared to Sears-Roebuck. Throw in a depression for good measure, promise a material heaven here on earth, Add a dash of anti-Semitism, anti-catholicism, anti-negroism, and a good large dose of anti-“furriners” in general and anti-intellectuals here at home and the result might be something quite frightening- particularly when one recalls that our voting system is such that a minority distributed as pluralities in enough states can constitute a working majority in Washington.” -Robert A. Heinlein (1953)