The guilt of original sin driving individuals towards redemption via organized religion thereby allowing increasingly large, stable civil structures in Europe.
Augustine and a clear exposition of original sin was in the late 4th-early 5th century, about four hundred years after the Roman Empire had already effectively integrated quite a bit of Europe and Asia into a diverse and sophisticated civilization that wouldn't be seen again for another millenia.
Around six hundred years after Augustine, Europe had William the Conquerer and the Holy Roman Empire, I guess? Five hundred years after that, Europe had the Renaissance based in a reclamation of the heritage of the ancient world.
I don't think original sin did a very good job of achieving the integration of anyone.
https://en.m.wikipedia.org/wiki/Original_sin says "The belief began to emerge in the 3rd century..." which is early enough to establish mild causality for Emperor Constantine. Remember, he didn't have to believe it only to suspect that it would be useful.
The whole thing was meant to be tongue in cheek. :)
