Nationalism, politics can unite people just as well, and unite them long-term.
No it can't not on non trivial time horizons. Nationalism and politics require a common cooperative foundation to sustain them. Inertia can only holds things together for a limited time.
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And belief in an imaginary friend will hold the society forever? LOL. You base this on what? The bloody history of the Christian church?
Those beliefs were imposed with quite a bit of fire and sword handiwork.
If I were you, I would worry about education and science to eradicate all the superstition and beliefs in the supernatural in few generations.
Science will be here in few hundred years, religions will not.
If you want to know where religions are going, check out the participation rates in younger generations.
http://www.pewforum.org/2018/06/13/young-adults-around-the-world-are-less-religious-by-several-measures/
The trend will accelerate as people become more educated and science fills gaps in our knowledge (that is where the God of the Gaps lives).
At the time of the Great Flood, there were only 8 people who still accepted God... Noah and hs family. When you consider the potential for reproduction back then, there were probably easily over a billion other people who didn't accept God. Who won?
