Correct me if I'm wrong, but wasn't Catholicism basically the default form of Christianity for the majority of the time there have been Christians?
Catholicism is as Christian as Mormonism is Christian. They both claim to be Christian, but their doctrines significantly contradict what the Bible has taught from the very beginning; the Bible has always been the cornerstone foundation for Christian doctrine and life.
In the Council of Trent, for example, the Catholic Church ruled that salvation is not by faith alone, but by faith plus a person's works, and that anyone who teaches it is by faith alone is to be anathema. In the Bible, Christ and his followers repeatedly taught that it is faith alone that saves a person (Ephesians 2:8-9, Romans 3, Romans 10, etc).
In Mormonism's 3rd Article of Faith, the LDS Church teaches that a person is saved by obedience to laws and ordinances, again a sharp contradiction of what the Bible has always taught.
It's basically like saying you drive a BMW just because you put a BMW sticker on a Mercedes. It may still be a luxury car and have a BMW tag on it, but it's not a BMW.
Something to think about: Isn't holding all Christians responsible for the actions of the Catholic Church the same as holding all Muslims responsible for CIA?
Yes, it is. I certainly do not hold all Muslims responsible for the atrocities Islamic fundamentalists commit.