Romans sealed their fate when they killed Caesar, and seeing as the Eastern Roman empire lasted a thousand more years after the West collapsed I'd go so far as to say Christianity made it last a lot longer than it should have. Rapid expansionism and over extending their borders also fucked them over, not to mention other Pagans like the Huns. They were also too reliant on North Africa for food as well.
Christianity and taking in immigrants just as our christian natiins still do to this day led to romes fall. Rome would still exist today if not for Christianity.
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