Christianity is one of the world religions along with Zoroastasim, Buddism, Hindu, Moslem and Bahia.
Christianity is itself divided into many sects and beliefs which makes it easy to accept that the Cathar faith was just another Christian sect.
In fact it was not it was something completely different.
Christianity took over and utilised the Roman Empire for its own ends and came to dominate Western Europe. From there it spread throughout the world as it was used by the various European nations as the distinguishing feature which separated them from “Primitive Peoples”