Well, the Trinity, of course! That's the first thing that comes to mind, because the doctrines of the Trinity as we understand them today developed very slowly within early Christianity. That was a LOT of debate about how and if God could have three aspects or incarnations.

The final details weren't worked out for centuries, and during the debates, there was quite a bit of violence against people who were on the "losing" sides of the arguments.

And despite how Christians tend to look back on things today, the eventual theological outcome was not foreordained.

Christians who lived just a generation or two after Jesus had wildly divergent views on the Trinity.