Asklemmy

The same can be asked about the Mormons. It’s Christians who have a weird obsession with deciding who gets to be Christian.

For me, if they believe in the divinity of Christ, they’re Christian.

There's plenty of great commentary here about why Christianity is divided up into different sects, but I think you're primarily interested in the narcissism of small differences. (https://en.m.wikipedia.org/wiki/Narcissism_of_small_differences)

Basically, if you've read about Dr. Suess' Starbellied Sneeches, you get the idea. Human brains are exceptional pattern recognition machines, and when a society is so homogenously Christian then those small differences become the cleavages along which identities form. That leads to things like Catholic / Christian divisions and the formation of the best joke in The Guardian history:

Once I saw this guy on a bridge about to jump. I said, "Don't do it!" He said, "Nobody loves me." I said, "God loves you. Do you believe in God?"

He said, "Yes." I said, "Are you a Christian or a Jew?" He said, "A Christian." I said, "Me, too! Protestant or Catholic?" He said, "Protestant." I said, "Me, too! What franchise?" He said, "Baptist." I said, "Me, too! Northern Baptist or Southern Baptist?" He said, "Northern Baptist." I said, "Me, too! Northern Conservative Baptist or Northern Liberal Baptist?"

He said, "Northern Conservative Baptist." I said, "Me, too! Northern Conservative Baptist Great Lakes Region, or Northern Conservative Baptist Eastern Region?" He said, "Northern Conservative Baptist Great Lakes Region." I said, "Me, too!"

Northern Conservative†Baptist Great Lakes Region Council of 1879, or Northern Conservative Baptist Great Lakes Region Council of 1912?" He said, "Northern Conservative Baptist Great Lakes Region Council of 1912." I said, "Die, heretic!" And I pushed him over.

https://www.theguardian.com/stage/2005/sep/29/comedy.religion

This might be a regional thing. At least in Germany, where the reformation took place, the term Christian include all groups, protestans, catholics, orthodox etc.

You won’t find me using this dichotomy—I just use the blanket term “Assholes.”

Because religion is all about dividing people into arbitrary groups. Catholicism is a specific type of dogmatic Christian theology

Because the Anglosphere was so xenophobic that even the Irish and Italians were too culturally exotic to them and thus demonized their entire belief system.

Because America is built by non-catholic faiths composed of heretics that love to burn women alive, abusers that craft abusive personality cults in hills-have-eyes country, false prophet grifters who tell you god's gonna cure your terminal ilness if you help them get a private jet for their dogs, the hate-worshiping demons that blight the south 'evangelicals' that drive modern american politics, and misogynistic polytheistic pagans wearing the Christian mask while dressed in their weird underwear and tell you you're not allowed to drink coffee but monster energy drinks are a-okay, who all want to be acknowledged as the real Christian faith instead of being labeled as schismatic protestants and chose to go with the common moniker "catholics & christians" instead of "catholics and protestants"

Every US president has been Christian but only one has been Catholic. It is considered as a clear distinction in the US imo