Most sects suffer from the No True Scotsman fallacy. there might be some ecumenical leanings when they’re dealing with the world at large, though internally, they usually don’t believe that anyone too far away from their particular flavor is really Christian-enough.
And that’s ignoring the rampant belief that no True Christian™️ can ever be whatever they happen to find abhorrent- abortion, democrats, lgbtq, the wrong race.
There’s plenty of evangelicals that don’t think Biden is a Christian- he now supports LGBTQ rights, is more or less pro choice, catholic. And a democrat. These are all things that evangelicals see as proof he’s not.
I’ve literally had one saying that certain pro-choice presidents weren’t really Christian, and that they persecute “real” Christians, because they need to feed the victim complex.