>>17796269 (OP)You want the harsh answer or the soft answer?
>Soft answer : Christianity was just universal, and had universal standards for a chance of an afterlife that didn't suck like Hades or Hel. People liked that almost unilaterally.
>Harsh answer : The religions of Europe at the time of the spread of Christianity were all spiritually bankrupt and none promised the theosis that Christianity proposed freely available to all men (most had you be born special to attain it, if you could attain it at all).
The "hatred of the flesh" that Christianity professed also wasn't something new in the minds of the intellectual elite of the time; the same was recognized by stoic movements across centuries, denouncing virtually the same thing as the Christians would without much change in the spiritual habitude of the average Roman.
christianity was a breath of fresh air in that it combined an already popular stoic viewpoint with a religious ritualistic lifestyle compatible with roman habitudes, so the highly legalistic and mystery-based helleno-roman cults were abandonned for the more mystic and yet stoic Christian faith.
Ironically, most of the Jews that didn't die in the Second Jewish revolt ended up converting to christianity for... basically the same reason.