This whole thread is based on a false premise.
Simply because Catholicism is a form of Christianity.
There are three major forms of Christianity, and those are Catholicism, Protestantism and the Greek-Russian Ortodox Christianity.
Within Protestantism, there are tons of different churchs, such as the methodist, the baptist, et al.
But comparing Catholicism with "Christianity"...

Puhleaze. That's extremely uneducated. After all, it was Martin Luther, Kalvin and their followers that left the Vatican. Not the other way around.
So if anything, Catholicism is Christianity.
Just ask yourself who the first pope was..

-Hedge