Sometimes religions and spiritual practices evolve through exchange whether it happens naturally or forcefully. Sometimes if you want religion to catch on, you have to package it so it is appealing. Christian holidays are established to correspond with pagan holidays or certain Christian figures reflect the loa in voodoo.

Christianity spread quickly because it was applicable for everyone at the time. Martyrs willing to die going against the Roman Empire as well as question some of the Jewish laws. It accepted everyone and anyone can become part of the religion. It really became more mainstream when Constantine, became the first Christian emperor as mentioned in other posts.