I think Christianity has spread as far as the Western World has allowed it to, its crossed many cultural divides as we see it in Africa, Europe, Latin America as well. There could never be a really dominant religion because of certain areas of the world have their religion and culture assimilated as one.
For example Islam in the middle East or Buddism in the East and Hinduism in India
Christianity - 33%
Islam - 24.1%
Hinduism - 15%
Buddhism - 7%
Particularly I think that Christianity spread thanks to the colonization of America, we must remember that the aborigines had no option, or they adopted Christianity to kill them, thanks for participating.
regards