For a belief set to remain distinct enough to copy itself, or at least allow its accurate reconstruction down through multiple generations - especially before the advent of widespread recorded media - it must be resistant to blurring. By this I mean, contain some internally driven self-referential logic that automatically leads the individual or group adherring to its system to deny almost point blank another conflicting system.
I know this is a little circular, self-fulfilling, but the criteria for 'scientific' testable/falsifiable critique is largely unavailable to religions to provide a proxy for dierct conflicts of words or deeds. (And never really would have been available anyway, or even desired at the time when such things would have made a difference). To remain distinct enough to become a mainstream religion, such a religious system of belief would always have to have had a 'this is the one true word' clause in their somewhere, along with a system of punishment for those who would embrace the changes an embracement or even sometimes a simple tolerance of another close, physically and metaphysically, system.
It would, to my mind, be even more unlikely that two mainstream religions like Christianity and Islam would become instant friends, than to have the current situation. It has to be this way.
I think that mainstream religions, to dip into symbolic evolution, are - after fulfilling their chief purposes of linking the reality/concept of God to humankind, or infinite to finite or whatever terminology suits - are also constructed to last.
The idea of God, or Gods is intuitive - it instantly draws parallels however consciously or unconsciously in the mind of the aspirant with paternity, less frequently maternity, and hierarchy, as well as an answer to the natural question arising from the state of being a technological ape - "I make things, what is made needs a maker, who made me..?" - ie the idea of God at its most simple, most naked, is almost instantly transferrable between location or culture simply by virtue of being communicated by humans.
There is also, in the times of primitive creation myth cycles, less hardcore reason for doubt. The "well it could be true" factor is much much higher when you literally don't know how anything, natural or unnatural, works. Think of how astonishingly difficult it would be now to create a wholly new God concept not based on any of the current accepted deities, and have it flourish.
So, we could reasonably assume that there used to be as many Gods as there were groups of people, back in the day, and that beyond the actual central concept of 'Supernatural being' they were extremely diffuse. Some whispy idea of your Dead Dad looking down and commentating on your hunting skills would suffice, because your Dead Dad didn't really have any other, more organized pantheon to compete with.
And it is the idea, and its implications, of inter-religious competition that I want to explore.
A deity without competition need not punish, nor reward, to establish itself. Intuition, imagination and boredom in any given group of people will suffice. As will any local hallucinogenic substances, and natural outflow from already ingrained ritual activity. People like to dance, sing and do things together. The reason matters less than the activity, at least in the beginning - novelty being its own reward.
But when groups merge or conflict, or even simply come within 'knowing' range, then so do pantheons. And let the best pantheon win. So how does a particular system of belief recruit some faith from its followers..?
I'm sorry to say that in my view, it is by using fear. Fear of death, fear of famine and disaster, fear of disease. But this hardly needs saying. But I think at least in its early stages of competition, religion does not need to up the stakes higher than simply temporal, mortal punishment.
It can also recruit more positive aspects by association, just like Coke recruits positive association from the latest crop of Popstars, or Scientology recruits it from Tom Cruise.
Groups become bands, and bands become chiefdoms and chiefdoms become states and states become empires. And with empires come Emporers. And with Empororers, or at least Augustus (Octavius) Caesar, come Empire cults, more political entities than religions but functioning as such all the same.
How can another religion however focussed, however resolved, compete with one upheld by the political and military might of an Emporer..? Easy, by doing the oone thing it can do, that Caesar cannot, and that is move the supernatural goalposts.
Caesar is mortal, and his power is necessarily finite. He has limits. What is the best he can do for you, what is the worst..? And what do either of these courses of action cost him..? At worst, Caeser can have you personally tortured and killed, and your friends, and your entire familly. At a push he could raze the place you came from and put your entire race to the sword - but you'd really have had to have pissed him off for him to expend so much resource on revenge. At best Caesar cold exhalt you beyond all men, save himself. But again, you'd have had to have done something inordinately just 'WOW' to have earned such a similar expenditure of resource.
And this is where supernature, which has no limits to its power save the imaginations of its adherrents, wins out. There are limits to how much pain Caesar can make you suffer, before you finally die. God has no such limits, especailly if you have a soul. And better still, this terrible ability to punish costs the temporal extention of God - the adherrent religious system... Absolutely nothing save invective and maybe a few engravings. Similarly, God has no limits on how he may exhalt you, which again, cost nothing.
But there is a flipside also. Because neither everlasting damnation, nor everlasting life, costs anything very much, at least on the part of the religious system involved, it can also be dispensed with impunity, which means, unlike the hard work you'd have to do in order to get Caesar to really seriously punish you, or the even harder work you'd have to do to get him to exhalt you, to recieve either punishment or exhaltation from a religious system you would hardly have to work at all by comparison.
Live peacefully by its central directives - get the gold rung - one far surpassing even Caesar's abilities to give.
Break a minor rule and risk absolute hell.
A truly supernatural religious system allows the underachiever to experience extremes they could only have drempt of prior. It opens Valhalla to everyone, and makes the heroes make room at their mead tables for the riff-raff, it opens heaven and puts angels, like common staff, on the doors.
After that, a religion becomes its own empire, aquires wealth and power in its own right, becomes kingmaker, rather than simply being dependent on kings. Ethereal reward and punishment for the masses, material and political reward and punishment for the influential few. Momentum.
Anyway. Sorry for the massive missive. Let's cut to the chase.
A religion can do all of the above things, the rags to riches tale, without having to be 'true', simply by being convincing, or at least being espoused by those who are capable of being convincing, physically or oratorically, and meeting the requisite terms of reward and punishment.
But still, there is more to religion than simply "Here is God. He is the true and only God. Obey him or else." All that is simply packaging - to borrow a metaphor from genetics - it is the transcriptory mechanism that copies the central message of DNA through the generations. The message of DNA constructs bodies of varying fitness in competition with the surrounding enviroment and the other bodies around it. The central message of religion constructs peoples of higher or lower populations, also in competition with those around it. The central message is a facillitator of the principle of coherrence.
To bind a group of people together, to be of one mind and one purpose requires more than biology can supply. It requires an intergrating set of Culturally and/or religiously driven morals, virtues or whatever you wish to term them, which provide a comon ground upon which for people who do not know each other very well to base mutual trust, in order for them to co-operate.
I'll admit to knowing very little of the intricacies of religious morality, but what I do know is that from old testament to new, revenge in equal measure turned to forgiveness and this I think, is key.
I know this from game theory. A social strategy which forgives, beats a social startegy that does not. It does so because a forgiveness clause allows two people, or two groups of people to overcome a mistake, a slight, a trespass and continue to work together.
And it is this commandment to forgive, driven home into the group psyche by the ultimate example of Christ upon the cross that is important, whether or not he was ressurected, whether or not he was the son of God.