As LambSlave points out, it wasn't just Christianity that led to success. IQ had a big part in it:
But IQ isn't the only factor. Another factor is culture. Early Christianity had a culture of pacifism. This was OK as long as non-pacifist pagan Roman soldiers were (mostly) protecting the pacifists from the barbarian hordes. While some Christians were killed for their faith, as long as you kept your faith quiet and paid your taxes on time, it was possible to live in peace.
This changed when Rome fell, and necessitated a new variation of Christianity which was fine with using force to protect themselves.
This changed when Rome fell, and necessitated a new variation of Christianity which was fine with using force to protect themselves.
Friedrich Nietzsche attacked Christianity for its pacifism (he equated loving your enemy and turning the other cheek with the moral codes of slaves). What he said was largely true of Christianity under the Roman Empire, but reading Nietzsche's writings about Christianity as a generalization makes you think that he missed the Crusades in particular and Medieval Christendom generally.