Well, it certainly hasn’t had anything like apostolic succession or a united hierarchy. Much like Judaism, there is no global center for leadership, and everything is kind of broken off to wherever people happen to be when it comes to local authority.
Also, I would argue that the differences between Sunni and Shia are pretty major. They both have the same Koran, but (again comparing to Judaism) there are other sets of teachings roughly comparable in importance and authority to that of the Talmud, however there are large bodies of teachings recognized by one group and completely rejected by the other. More to the point, the content of these disputed teachings has much to do with eschatology, the Messiah, and the role that Muslims play in getting from A to B. Very important stuff, certainly far more serious differences than many of the heresies dealt with in the early church.
There really is quite a bit of variety within Islam, although admittedly it is (overall) the most monolithic of the major world religions. I would say that’s mostly because Islam has a history of pretty vicious coercion, especially when it comes to regulating the beliefs and practices of those who are born into Islam. If anyone wants to go do something different, they are not just cast out of Islam- they are forcibly silenced, prevented from infecting anyone else with their errors, or just killed. And in most of the majority-Muslim countries in the world today, that type of coercion continues to happen big time. There is quite a bit of variance from country to country when it comes to public opinion about Sharia law as it relates to non-Muslims, but there is always some degree of coercion when it comes to those who were born into Islam.
Catholicism used to be just like that. Basically exactly like that, the era of Christendom can easily be described as Christianity’s version of Sharia law, and the coercive behavior was present in different amounts at different times, depending on who was in charge and what was going on. Of course, Islam has done all of this without a global hierarchy and they’ve even done it without any concept of divine tradition or divine teaching authority. A mullah is not at all like a priest or a bishop, a mullah is exactly like Islam’s version of a Protestant minister who might be really smart, maybe not, and might have gone to some really good schools, but then again maybe not. Except in Islam there is nothing like a priest or a bishop to compare him to, they are nowhere to be found. (Edit- except in Christianity, of course. What I am saying is that this concept does not exist within Islam. Just imagine if the entire concept of sacerdotalism was absent from the entire Christian religion. And oh how disunited that would be, except maybe not as we can see with Islam).
I do think that as you look into this a bit more, you’ll see that there is more fracturing within Islam than you are initially giving it credit for. But I do agree that it’s been held in check to a greater extent than in Christianity- and that is mostly because the Reformers didn’t let the Catholics get away with violence and coercion to the extent that they would have liked…the Enlightenment was a major thing that never really happened in the Muslim world, that’s certainly worth mentioning…and Islam continues to have Sharia law, it continues to coerce where it can and that happens to affect the vast majority of Muslims in the world today.
If the Reformation had failed, if the CC had been successful in making Protestantism illegal and then enforcing those laws effectively, if major aspects of the Enlightenment were stricken from the historical record, Christianity would probably be at about the same level. And that would be a terrible thing, if you ask me. Freedom is a valuable thing, and unity is less than worthless if it’s achieved through coercive means. And that is exactly what the Catholic Church was trying to achieve in its fight against Protestantism.
I do have a question of my own that I’d like to counter with, though. How does the unity of Islam compare to the unity of specifically Orthodox Christianity? If you exclude the Catholic Church and its Protestant issues, and if you minimize the importance of the Schism in the same way that you’ve minimized the Sunni-Shia rift- what sort of comparison are we making if we’re looking at Islam alongside Eastern and Oriental Orthodoxy, taken together? And a follow-up: What if we just compare Sunni Islam to Eastern Orthodoxy? How do you suppose that comparison would go?
Please keep in mind that Sunni Islam does not have a teaching hierarchy- rather, what happens is a regular gathering of all the most prominent religious Sunni leaders and there, they talk about what they can all agree on and what they can’t all agree on. They make an effort to agree on as much as they can, and they hope to influence each other in a way that they can achieve a little bit more consensus each time they get together. So that’s their process, and assessing just how much all these Sunni leaders agree with each other is a pretty complex matter that can’t be easily summarized in a sentence or two. Within Sunni Islam, there is of course much that is pretty monolithic but there are also a bunch of recurring issues on which common ground and consensus is very difficult.
But it’s easy for this to fly beneath the Christian radar, especially if it’s news to you that they meet like this and go through that process at all. And for what it’s worth, Evangelical Protestants have a similar process of seeking consensus but Catholics give us zero credit for having any real unity with one another. Yet here you go talking about Sunni Islam, and despite these striking similarities in teaching authority and the process of achieving consensus, you’re giving them all the credit in the world for having fantastic unity. The truth is more complicated in both cases, it lies somewhere in between.