It’s certainly a hard question to answer, that’s for sure!
My own view on the whole culture/religion thing is that religions do tend to follow along existing cultures, even ‘invading’ religions can pick up local traditions. Christianity is an excellent example, if you look on how it was adopted in the Far East it is markedly different to Western Europe. There are also huge differences between the Greek Orthodox and Catholic churches. You’ve seen it metamorphise over time to keep up with current culture into the ‘off-the-shelf’ version we have today in ‘modern’ countries where there are so many different sects that you can choose one that fits your particular ideologies rather than having to conform to a strict interpretation.
Even Islam, for which the original text is supposed to be avialable to us, has not been immune to this metamorphising force of culture, in the West moderate Islamics tend to drop all the wife beating passages or at least sanitise them so that it fits witrh the current cultures ideology.
It is also quite obvious looking at the world today that when a culture that has been heavily influenced by religion feels threatened, by so called cultural imperialism [1], it revives a traditional, or fundamental, form of religion to try and hold on to its culture. The two become inseperable.
However religion is like a virus [2], it mutates even without external influence. The more ‘virulant’ the religion is, the more likely it will be believed. Hence the move away from polytheism to monotheism, from the malevolent Gods (Roman/Greek) to the righteous God (Jewish) to the benevolent God (Christian). This is a very broad example and should not be taken too seriously however.
I must admit, as a final point, that I’m heavily influenced by Dawkin’s idea of memes, ideas that replicate themselves in people’s minds. The more attractive the idea, the more likely it will be that it imprints itself on the collective consciousness. Hence the more ‘attractive’ the religion, maybe in terms of post-death benefits or how well it reflects particular moral systems, the more likely it will spread.
This reply is perhaps not what you were looking for, I’ve looked at religion more from the perspective of the idea behind it. My intuition is that a cuilture will only adopt a religion if it mirrors their ideology in some way, though I think the whole process once past a religion’s conception, perhaps the part you were asking about, is reciprocial, culture changes religions and vice versa. That is more how I’ve responded.
But maybe it will bring some more replies on your question, which I must say I thought was a good one, if difficult to answer, especially as it is not my primary area of philosophical interest.
[1] Personally I dislike the connotations of the term imperialism, I have argued in “Iraq poll updated” that there is nothing imperialistic about it all, at least not till the last 5-10 years when it was adopted as American Foreign policy.
[2] I hope I don’t offend too many people. Just take as said that your religion is obviously not like a virus as it is the one true religion and all the others are the virulent evil ones.