I like how this thread brings up the question of whether or not human beings have a difficult time accepting other ways cultures could develop in theory, and the following posts all answer "Yes!"
The problem here is that we don't have any examples to draw on other than our own. It's all pure conjecture at this point. I read every single theory posted in this thread, enjoyed reading them, and think they are all very creative and imaginative.
Personally, I do believe we are moving towards a homogenized global culture that will likely speak the same language, use the same currency, be much more 'racially mixed' than we are today, etc. I don't believe that faith will be universal in the next 300 years. Although we're nowhere near a complete recipe at this point, humanity has unarguably been intermingling more with itself in the last few hundred years, due to advancements in communication and transportation. If we continue on the same track, to me it's fairly logical to assume that we will end up as a generally homogenized single culture in the next few hundred years, possibly much more so in the future beyond that.