Alceste
Vagabond
Everyone makes a supreme authority. The difference is that some people make it themself, some make it G-d, some make it a different G/god(dess) or several, some make it...Chuck Norris, I don't know! We all find the authority that suits us best.
I haven't got an authoritarian bone in my body. I bow to no-one and would be faintly disgusted if anyone presumed to bow to me. So, there goes that theory.
The point I'm trying to get at (and I would do a better job if it weren't so late) is that I think when you attribute too much to culture and upbringing, you're cheapening (even if only unintentionally) the religious experience of others. There's the implication that people who follow the beliefs of their part of the world, they aren't fully considering their options. Now, some of 'em aren't, to be sure. I think that you can't attribute too much to cultural influences. Tangentially related is the point that we all develop belief systems that fit our conceptions of the world, regardless of whether those belifes or that world includes a god.
Here, let's be scientific about this. Let's say your hypothesis is that people reach their religious beliefs after a thorough period of soul-searching where all the alternatives are carefully considered, and religion has nothing to do with culture or upbringing. (This is to take one extreme position - not to misrepresent your post).
If this were the case, what evidence would you expect to see?
If religions were the result of nothing but soul-searching, we would expect to see a random geographical distribution of religious beliefs, and that these beliefs would be only weakly correlated with ethnic / cultural identifiers such as language.
Alternatively, if it is the case that religion was ENTIRELY the product of culture and upbringing (the other extreme), we would expect to see a geographical distribution of religions that correlates neatly with other cultural identifiers such as language.
Assuming these expectations are reasonable (and I think you'll agree that they are), we can have a look at how the world actually is and establish (roughly) the ratio of upbringing to soul-searching in influencing religious beliefs.
In fact, we see strongly concentrated geographic distribution of religions that happens to correlate very strongly to other cultural indicators. There is a little bit of "spillage" - Christians in Buddhist cultures, Buddhists in Christian cultures, etc. but these are generally very marginal in any given culture.
Given the above, it's safe to conclude that religion is strongly correlated with culture and upbringing and weakly correlated with individual soul searching.