Christianity as a practiced religion is real. The figures in Christianity which Christians worship are not, at least we have no hard evidence of them being real.
I mean a lot of concepts in economics are descriptive, they note phenomenon that just happen when humans engage in trade or resource allocation amongst each other.
It's basically the three body problem but with sharing according to the maximum satisfaction of want and need instead of gravitational fields.
Kind of both. The science of economics is full of models with false assumptions and self fulfilling prophecies. Economy exists because we believe it does and because of said self fulfilling prophecies
Economy exists not because we believe it does, but because we agreed to follow particular rules, many of which are enforced by state. It is emergent phenomenon. But it does not make it any less real than any other emergent phenomenon, like air pressure.