With a POD not earlier then the XIV Century, make so that a major religious reform starting in Southern Europe ("Southern Europe" in this context begin Spain, Portugal, Italy under the Arno and the Catholic parts of the Balkans) ends up in dividing (permanently) Western Christianity in two halves.
Can start like OTL's Protestant Reform from a single man condemning a certain practice (bonus points if it's something besides the indulgences), or as a group effort of a single or multiple church communities.