MAJOR differences between Catholicism and Christianity? That makes no sense. Catholicism is a form of Christianity (the original form, you might say). What you mean is that there are major differences between Catholicism and the multitude of different forms of Protestantism -- some of which are fundamentalist, literalist, and wacky.
Christians are from the South, where Jesus lived. Catholics are filthy liberal Yankees. #Don'tAskAboutTheJewsOrMooslims