Great point!
Protestant Christianity didn't begin until the invention of the printing press. The bible, until that time, was only available in Latin and only to the clergy. Thus, whatever the Catholic church said was in the bible was all that anybody knew. It's just as well, because the average person on a feudal manor was hardly educated enough to read it even if it was in his or her own language. But with the printing press came the ability to check out what the bible says, and many people (Martin Luther notably) believed that the Catholic church was following a religion of their own invention rather than one based on the bible that people could now read for themselves.
I used to be a protestant (now an atheist), but I don't see why a Catholic couldn't be a Christian in the same way protestants are. "Believe in the Lord Jesus Christ and thou shalt be saved...", and surely Catholics believe in the Lord Jesus Christ. I don't see why a difference in rituals ought to separate Christians, although I don't understand either why a difference in rituals ought to exist when one can supposedly ask God how He wants to be worshiped.