The Collaborative International Dictionary of English v.0.48:

Christianization \Chris`tian*i*za"tion\, n.
   The act or process of converting or being converted to a true
   Christianity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Christianization
    n 1: conversion to Christianity [syn: Christianization,
         Christianisation]