Frankly, I don't think atheists give a damn what you know about Mark Twain.
Fundamentalist Christians HATE Mark Twain, and I was never sure why. Twain wrote a lot of passages criticizing the Christians of his day for their sanctimoniousness and arrogance. Huckleberry Finn, for instance, had an internal fight between his religious teaching and his conscience!
I have discussions with Christians sometimes about The Devil. Christians believe if the Devil showed up on earth, masquerading as a human, they could tell him in an instant because he'd be really ugly and his evil would be obvious. But lots of authors have written fanciful stories about the Devil showing up and being very good looking and very persuasive, and unsuspecting Christians loved him and followed him believing he was good, and that was how he was able to mislead them and corrupt them. Even CS Lewis used this theme in Screwtape Letters.
Mark Twain wrote a story like this, and I'm sorry I can't remember the name of it. I always wondered if fundys badmouth him because of that story. The theme suggests that people of faith are easily corrupted through their own faith, IOW that strong, unquestioning, unskeptical faith can be harmful as well as good. Just for example, Mark Twain was appalled that Christianity in the South supported slavery!