The first problem is people like to make the term as inclusive as possible which is unreasonable without an a priori definition. As with any popular subject you can likely find an argument for just about anyone being one. That keeps academics busy. For example the Declaration of Independence was an obvious prerequisite to starting a country but the country is its document of unification otherwise it was still thirteen independent nations. To keep it as narrow as an non PhD thesis would require it should be limited to the constitution in force today.

In that case the term applies to the senior political thinkers. Washington wasn't one of them. In fact he wasn't much of a political thinker at all. When limited to those who participated in drafting the constitution, Jefferson was in Paris at the time. The Federalist Papers are generally considered the minimum required reading after having understood the meaning of the parts of the constitution itself. There is so much popular nonsense understanding the constitution can be considered an exercise in unlearning first. Without a familiarity with the Federalist Papers there is no basis for considering much of anything else. Reading about them helps to point out what is not in them which is not apparent from their form as pro-constitution arguments. There were also essays against the constitution worth reading.

As to actually understanding the constitution the Supreme Court often splits 5-4 on what it means.

As to Christianity as part of the political thinking going into the Constitution, the short answer is none and the long answer is none. Fundies claiming otherwise are lying, ignorant and more likely both. Newton had a degree in divinity. Christianity did not play a part in his work either. Its importance in their personal lives is debatable as most were Deists which doesn't doesn't have any use for religions or religious doctrines. There was a god who started things then washed its hands of it immediately. Given their times they did not write anti-church essays. Combing through their works you can find a few things. If I might attempt to describe Jefferson he appears to have perhaps artificially elevated Jesus to point out the churches were not based upon his teachings.

With that in mind it is necessary to separate the idea of organized religion from ideas of a god and Jesus. Consider Franklin does appear to have honored the ideas of the Quakers except that adultery part. A little manly adultery was expected. He was playboy.

If some group at Madison is trying to claim otherwise, refuse to engage in argumentation and insist upon seeing the words they are talking about in context. Watch for all the logical fallacies such as presuming the conclusion in "what else could he have meant?" and all the rest of them.

As to the words themselves, I have had people ask what they presume is a rhetorical question to the effect, If there is no religion why does the oath of office say swear to god? It not what you don't know it is the things you know that are not true which cause the problems.