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