I tried to explain the problem a while ago by explaining that most of the factors involved are secret societies -- to put it in a more modern context, it is like a secret cell of Soviet spies in the U.S. government hiring North Vietnamese and Cuban infiltrators to find out if the Red Chinese embassy in Japan is really spying for the Lithuanians in an attempt to find out if the KGB was behind the plot to kill the Pope and hire more Afghanistan refugees to doublecheck the rumours about the John Birch Society joining forces with the Mafia to break the stranglehold the Teamsters have on the underground network of solidarity supporters in Moose Jaw.
He goes on to note that "300 issues may not be enough" and as it turns out his inability to portray this effectively (in the same intro he says he deliberately chose to do "High Society" partly in attempt to deal with all these things, and also says that as he's writing this he's half done with #50 and it has "one helpful line of dialogue"), combined with his eventual waning interest in the fantasy/adventure/conspiracy plot compared to using Cerebus as a vehicle to comment on contemporary reality meant that we would never really be able to make much more sense of it all than one can of that analogy.
Still, for what it's worth, here is something like a scorecard with the players in this high-stakes game:
The Eastern Orthodox Tarimite Church: headed by the Eastern Pope, who must be a native Iestan, this group is apparently the successor to a matriarchal society that once ruled Iest, though it's not certain whether they worshipped "Tarim" or "Terim" (see Oscar's discourse on Guffins in "Jaka's Story"). In its structure and socio-political role in society, this seems more-or-less based on the Roman Catholic Church in the late Middle Ages, just before the Reformation (in other words, at about the time of the Italian Renaissance, our 15th Century, which is roughly about the same time it is in Estarcion during our story), but in doctrine it sometimes seems more like the puritanical Protestant Churches like Assembly of God that have produced most American televangelists.
The Western Empire: headed by an Emperor/Pope in one person who rules by right of conquest, literally killing his predecessor in order to claim the title of Pope. Other than that bit of weirdness, this church is similar in doctrine to the other Church of Tarim, but even more puritanical and less tolerant. Think of the leadership of the Eastern Church as Jesuits, intellectually flexible and capable of almost atheistic devil's advocate positions in argumentation, for instance, while the Western Church is more Calvinistic and dogmatic.
The Cirinists: led by an aardvark who calls herself "Cirin" but was born "Serna." There are hints that there was an earlier Cirin, perhaps even multiple earlier Cirins, as there have been many people named Suenteus Po. Certainly Cirin (the real Cirin, Serna's friend and the real founder of the movement) was not the first to envision a matriarchy. As I pointed out earlier, Iest had once been ruled by a matriarchy, and early on ("Mind Games II," issue #28) Po says that Cirin is trying to "restore" the Corn-King rituals. The Cirinists are, like the Tarimite, monotheists, but they believe that God is Terim, a Great Mother, rather than Tarim, a Great Father.
The Kevillists: founded by Astoria in rebellion against the Cirinists, they share many of the Cirinists core belief (such as worshipping Terim), but reject the rule of Mothers and seek freedom for Daughters, including the freedom to reject Motherhood through birth control and/or abortion. In many ways, Kevillists mirror modern feminists, as Dave sees them.
The Eye of the Pyramid: a group supposedly run by Lord Julius' social secretary and taken care of during the Palnu Trilogy (issues #14-16), it later resurfaces and seems to be in fact an organization run behind the scenes by Astoria and her Kevillists, using bureaucrats and secretaries to infiltrate and undermine governments.
The Illusionists: the movement founded by Suenteus Po the First, about whom we learn very, very little through the course of the story, and yet whose way of thinking, what little we can discover of it, seems to be largely in line with Dave's own up until his conversion to monotheism. I'll explain shortly.
Weisshaupt's minions: these must be quite numerous by the time of the climactic Trial in "Church & State," when even after death he is able to place a gold sphere in the Papal Throne Room. Weisshaupt may be an Illusionist. Cirin certainly thinks so, and Pope Harmony calls him a Magician.
Dave has recently stated categorically that Lord Julius was an Illusionist, although it's not certain whether that is the monotheistic Dave looking back on what he has created and giving his opinion or that is what he had in mind from the beginning. Certainly "Mind Games II" discusses the possibility that certain political leaders in Estarcion may be Illusionists, and at the time Julius was the most prominent such leader that we knew.
There are presumably other factions we know even less about. Sir Gerrick, for instance, must have his own group of followers, and Theresa, though not an entirely reliable narrator, tells of factions within Cirinism other than Astoria's nascent Kevillism and Cirin's attempts to control dissent. However, these are the main branches that are interwoven into the story that is Cerebus, only to be dropped precipitously two-thirds of the way through (except for the triumphant but somewhat changed Cirinists).
The two Tarimite Churches are contrasted with the two movements that worship (or at least celebrate) Terim, but the joker in the deck is Illusionism. We learn less about Illusionism than we do any of the others, and its fundamental tenets remain elusive, but I believe that in many ways it is closer to the outlook Dave Sim had at the beginning of creating Cerebus than any of the other available views of the world.
Why do I believe this? One reason is Suenteus Po. Although it's clear that he is a character in his own right and not just a mouthpiece for the author, it's also clear that Dave uses him not only to deliver information to the reader through his conversations with Cerebus, but also to impart a certain attitude about that information which I think in general Dave shares.
Of course, when we first meet Suenteus Po he seems to be a not-very-bright stoner, more like Sean Penn's character from "Fast Times at Ridgemont High" than the founder of a religious-social-political movement. Less than a year later, he's having a high-level discussion about political strategy. These two personalities seem contradictory, indeed I've argued that Dave's view of Po fundamentally changed in between them, and yet to some extent they do both represent Dave himself, during that period in his life (well, except for the "not-very-bright" part, and any of us can seem not-very-bright when we're stoned). And Po would, I believe, would continue to be more like Dave than any of his other characters through the first 200 issues. When Po tells Cerebus the best way to live is to live simply, I believe that is a decision Dave had already come to, and has continued to believe. Indeed, in many ways his personal lifestyle now is even more like Po's.
Another reason is that I don't think Dave was ever sure just exactly what it was he believed. Po tells Cerebus that Illusionism is a discipline, which Cerebus later in the conversations restates as a "way of thinking," but we never get a really good idea of what that way of thinking really is. Cerebus says that he already has his own way of thinking, one presumably different from that of the Illusionists, but of course he has studied under Magus Doran, understands what Po means by "second meditation," and the difference between "complex structurally or manifest?" so it's not certain whether or not his outlook is all that different from Po's at the time of "Mind Games II." It's true that when Cerebus is asked what he believes and responds that it is easier to see during the day than during the night "and not much else besides," Po responds in turn that there are other things to believe in, but it's also pretty clear that Po doesn't believe in Tarim or Terim, or "God" as Dave would later come to conceive Him, either.
What *do* Illusionists believe? Or believe in? We never do know for sure, and the hints we get are mostly in "Mothers and Daughters," which I haven't gotten to yet in my reread. But one reason for that may very well be that Dave thought of the Illusionists rejection of Tarim/Terim as being "right," because he himself rejected the standard traditional religious concept of "God." However, since he didn't really know what to replace "God" with -- and he *did* believe that there was, as he quotes Neil Gaiman later, ". . . something there" -- he never really figured out exactly what the Illusionists believed.
Or not. In the long run, it's too simplistic to think that Dave ever intended the Illusionists to have "The Truth." The Dave who began Cerebus didn't believe in The Truth as a concept, or at least didn't believe that one could ever know what it was. To some extent, this is still true, as evidenced by a recent Blog & Mail entry (#106, December 26, 2006), where Dave quotes and then comments on a Mormon Devotional someone gave him on his trip to Salt Lake City:
There are but very few beings in the world who understand rightly the character of God. If men do not comprehend the character of God, they do not comprehend their own character.
How can you say things like that with a straight face without adding "in my opinion" or "it seems to me"? First of all, whoever you are, you don't know and can never know more than a handful of "beings in the world" so how can you say definitively whether or not they "understand rightly the character of God"? You're making yourself into a judge of people that you've met and their innermost awarenesses and stating definitively that you know whether or not those innermost awarenesses are accurate or inaccurate. Excuse me, but that's, at the very least, extremely presumptuous.
So even though Dave now believes he has found The Truth (and despite this disclaimer does in fact himself often say things in the same dogmatic way he is criticizing here), it's clear that he still believes that the search for Truth is one that is never going to be fully completed for anyone, that certainty is not available to anyone in this life.