The fact that the Religious Right cherry-picks through their other claim to authority - the bible, and ignores the rest - while claiming the book, as a whole is the ultimate source of truth, shoves the notion of surprise out the window with regards to their treatment of other books, speeches, reality.
Their arguments are nearly always constructed as an appeal to authority. The bible said it, so it’s true. Jesus said this, so it’s true. Founding Father said {blah}, so {blah} is true. Thing is, authority isn’t absolute, so all appeals to authority which assert that the authority is necessarily infallible are fallacious. They rely on the dance of syllogism - discourse in which, certain things having been supposed, something different from the things supposed results of necessity because these things are so. In the RR’s faulty logic - US is a Christian Nation ™ because Washington said {insert out of context pro-Christian quote here} and Washington was a Founding Father {insert implied infallibility here}. The syllogism doesn’t have any greater weight if it uses the same faulty argument, only with the conclusion that - US is not a Christian Nation ™ because Washington said {insert out of context pro-deist quote here} and Washington was a Founding Father {insert implied infallibility here}.
The deductive argument is invalid because the conclusion does not follow from the premises in the manner described. It may be true that Washington said such-and-such, and it may be true that he is a founding father; however, simply because he helped found a country and, on occasion, said things does not mean that county is/is not Christian on this basis.
I cringe every time I hear the words “founding fathers” because I know some stupid utterance will follow. To me - it’s the same as using a celebrity in advertising. Martha Stewart says that Febreeze is the best, and she, as a domestic diva, has the authority to discern what is the best, so you should buy it. Reality - some other product is equality good or better, and therefore the appeal from authority is fallible. The Christian Nation argument is the same don’t-question-authority drivel I expect from that lot. Founding fathers are the authority on the founding of the nation, therefore their words regarding it are unquestionable. RR Mission - find a founding father whose quotes align with the thing they want to assert, proceed to growl at anyone that questions said authority.
So - why, when I recognize this appeal to authority is faulty reasoning, would I engage in the same sort of reasoning to prop up my side? I would rather structure an argument about the secular nature of the US government that gives due consideration to the evidence, the context of judgment, the relevant criteria for making that judgment well, the applicable methods/techniques for forming that judgment, and the applicable theoretical constructs for understanding the nature of the problem and the question at hand. This argument requires equal critical analysis from the opposition to attack the argument - but does not hold said argument as immune from criticism because it is rooted in the because-so-and-so-said-such of arguing from authority.