Some of the most fanatical fundamentalists are those who describe
themselves as
atheists. They technically are, but they are religious, its just their religion doesn't have a God (unless you count Hitler as some sort of anti-God)
At this point the word "religious" is not part of my vocabulary when I talk about people of faith in God. Because at best it's meaningless in that context; at worst, it's derogatory.
There is a word used to describe certain Christians, "evangelical," which means strictly, according to the (Christian) Gospel, and a little more nuanced, referring to Christians who hold firm the authority of the Bible and a personal conversion to faith. A number of Christian denominations--perhaps not Catholic, Orthodox, or some protestant mainline--could be described this way. But in public, it's a stain. It's the evangelicals who (used to) terrorize abortion clinics. Evangelicals had a big hand in voting in Trump, twice! Evangelicals hate gays. I mean, that's what they say. But none of it is actually true. It's a misused label for conveniently grouping people together who don't necessarily belong grouped together.
So, even though it's a fine word with a solid definition, it's got baggage, like "religious" and I wouldn't use it among most company.
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* QWK2WWIV * Back to the Future (bttfbbs.com)