Christianity - One Religion? I Think Not.

[The Velveteen Ocelot]

1. "Christianity" covers a whole lot of variations.

And it has ever since the very beginning. The prevailing denomination will designate often designate groups holding variant beliefs as heresies. In the early years, once the Catholic Church became the "official" religion under Constantine, the Council of Nicaea decided what the "correct" doctrine should be, and anything else was heretical. Here's a list of Christian "heresies": https://en.wikipedia.org/wiki/List_of_Christian_heresies These days most people don't worry about being called heretics because the Catholic Church no longer can do much besides complain. I know a number of Lutherans who think Mormons are heretics, though Lutherans were once considered heretics themselves. In this country the Baptists really got schismatic, and in just about any small town in the South there would be multiple Baptist churches, all of which claimed to be the only true religion. I think it has a lot to do with people tending to be tribal and intolerant, whether it's about religion or anything else.