Monotheist religions are typically mutually exclusive. Even Christians will tell Jews that they don't believe in the right god, because Jesus Christ, as a part of their god, isn't recognized by the Jews. Muslims, contrary to popular Christian belief, do include Jesus in their theology: to them, he is a great prophet, but he is a son of god in the same way that all people are the sons of god. That seems to put a damper on the possibility of agreement right there, no?
But why does it have to be that way? Scholars will (hopefully) agree that the Muslim faith is an off-shoot of the Judeo-Christian traditions in the first half of the first millenium of our present era. Christendom itself is considered (though now in a kind way) as a sect of the Jewish faith. (In case you're catching up, Jesus got in trouble with the Jewish authorities of his time for claiming something which was heretical within Jewish circles, but became orthodox theology to his followers: namely, that he was an aspect of god.) These three faiths (and I use them only as examples of how this may happen) all share the same root-God: Yahweh, Allah, "God."
Will those of these faiths agree? "They don't know the whole truth." "They have been misled." "They are the lost ones." "Their god is a false god." The reactions vary, but are generally not favorable. If their god is also your god, why are your religions differentiated? To be more specific in our example, we can look at Christianity: what are called denominations would just as well be called sects, except that, other than perhaps two forms of Catholicism, we don't have a "base" religion with which we can compare the differing groups. Some groups might even tell you that catholics will not attain salvation -- or that mormons worship the wrong god and are damned to hell because they've added a book to their canon, the Book of Mormon.
I'll step out on a limb and assert the following: we define god by attributes, specifically attributes revealed to us by history (of one sort or another.) Allah is not Yahweh because their histories and the beliefs of their faithful (about them or not) differ -- not by the essence of their being(s), about which we likely know very little, if anything.
What does that say about denominations? It says that if they disagree on more than the most basic beliefs, they believe, in effect, in a different god. As the group already maintains a belief structure incompatible with other belief structures, it is highly probable that these people will believe they will be the only ones to gain favor from their god, including salvation and eternal life (if such things are part of the bundle.) It's not uncommon to hear baptists muttering about methodists, or lutherans about some other group: the reason they form those groups is that like-minded individuals (agreeing on the details) have joined together, separating themselves from others.
There is a trade-off at this point: religious leaders can attempt to teach "the basics" to a broad base, and hope that this is enough, or they can believe that every last detail of their system is necessary to a "good" faith, and spend much more time teaching these values and lessons to fewer people. The fewer details are included, the higher the likelihood that two faiths will become just one; many, however, are not satisfied by a simple religion of a few basic rules. There is a need for secrets, for mystical lack of understanding, for something more to thirst after all the time. If it were easy to understand and easy to practice, how could it be of any benefit? (Ever heard the phrase "no pain, no gain?")
How much detail is needed? There are basic things I know certain denominations of christians would say are necessary; but they would rather teach you much more than this. Why? Beyond the basics needed to assure the new believer eternal life, do the details matter? You'll be told, most likely, that this all makes your earthly life better. That same earthly life that, time-wise, is a speck compared to the eternal life you've just been guaranteed by your faith. Is there anything wrong with teaching details?
There are dangers that arise from a zealously detailed faith. Hopefully, our eternal life (if there is one) won't be judged by the details of how you take the eucharist -- but by creating denominations and sects through disagreements over small details, there's a high chance the heresy that will creep in once the groups have separated will be more significant than the things that originally divided the groups. There's a similar process in the theory of natural selection, allowing speciation: members of a flock may get stranded in different environments, and the slight differences among the two groups lead to larger ones, until one day they've become thoroughly distinguishable (and unable to breed -- in this case, take communion together without attempting to convert each other.) A lot of time is spent worrying about the details of faith, when possibly larger issues are at stake. The nit-picking can devolve into outright fights, which rarely look good for the group as a whole to anyone seeking to find absolute truth and peace of mind (security of belief) in religion. Piling belief on top of belief, building a larger structure, is a bit like a house of cards: there's a chance that somewhere along the way a detail will be introduced that, for one reason or another, disturbs the believer; the rejection felt can snowball into rejection of the entire belief system, effectively deconverting the convert. It would seem prudent to introduce new intricacies slowly and carefully, unless they are necessary to the person's eternal salvation.
Must religious leaders cater to the market of beliefs, by allowing niche beliefs to develop, leading small groups to become free-standing denominations of their own? Can the religions that teach love not also teach tolerance of at least minor differences, such as the method with which to baptize believers? On the scale of eternity, will any of it matter?
Ed. 7/2/2003 :: I believe Ensis may have overestimated the reach of my point: I would not, in fact, say that all of the religions mentioned here should join together. I would, however, say that we should take a good look at what divides us, and decide on a case-per-case basis whether or not it really matters. We're all entitled to our own opinions, of course; we shouldn't feel compelled to avoid topics simply because they divide us, or to reach concensus for the sake of concensus. A world of one-man religions, however, seems rather trite.
As to the nature of god: at no point did I say anything conclusive about this; I only stated that the religions mentioned seem to have grown out of each other, initially starting out as parts of the same religion. As such, it seems unfair to compare them the way we would two entirely different religions, born in different times, of different cultures, with entirely different ideas about their god(s). Christians see their deity as enlightened followers of a still-existing religion, just like muslims do. Jews believe the other two to have taken things too far, to have added too much story, to have perverted the truth. We cannot ignore the history of these splits for the sake of establishing one view of god as the only valid one -- at some point, the followers of each of these religions believed they simply saw the same god differently (and more correctly.) That's not a small matter, easily ignored.
Personal identity is already a problem in our world today: twins can switch places, thieves can take control of your bank account. We rely on external aspects of a person to know them and identify them; we could easily have a common friend, but not realize it because we focus on different things when in the presence of this person. I might describe the friend as interested in sports, and a funny guy; you might describe the same person as studious and sarcastic. Is it the same person? How will we know? Is it a different person only because the outside aspects seem different? Is it the same person, again based on outside facets? It's apparent that it's at least plausible we could be mistaken about the identity of a potentially common friend.
The problem is that monotheistic religions only allow for one god (by definition) on a absolute scale: if nothing else, all monotheistic religions speak of the same god, whether they know it or not. They may be speaking falsehoods, in the sense that they misrepresent the only entity they could possibly be talking about, but there's no real choice in the matter. By saying that our gods are different because we see them differently (through the eyes of history and personal experience) is like saying that our common friend doesn't exist, because we see him differently. It is a statement we cannot assert safely. Is your god my god as well? Read Ensis' article(s) and see what you think.