The ultimate factor in the rise of Christianity?  It's ideas:
"The simple phrase "For God so loved the world," would
have puzzled an educated pagan," (sociologist Rodney Stark)
says.  "The notion that the gods care how we treat one another
would have been dismissed as patently absurd."

To the Greco-Roman world, mercy was a character flaw; the
ideal was justice.  "Since mercy involves providing unearned
help or relief, it was contrary to justice," Stark adds.  "This
was the moral climate in which Christianity taught that mercy
is one of the primaray virtues--that a merciful God requires
humans to be merciful.  Moreover, the corollary that because
God loves humanity, Christians may not please God unless
they love one another was something entirely new. 

"Perhaps even more revolutionary was the principle that
Christian love and charity must extend beyond the boundaries
of family and tribe, indeed that it must even extend beyond
the Christian community."

--Paul Galloway, "How Jesus won the west:  Christianity became
dominant because it offered better ideas and unexpected mercy,"  
The Lutheran, November, 1998,19.