Why do people keep saying Catholicism isn't Christianity?

Last time I checked it was the first major form of Christianity around, after successfully wiping out different Christians sect that didn't follow the same precepts, such as the Gnostics, Arians, etc. possibly wiping out the true meaning of Christianity. but that is a story for another time :) All other forms of Christianity are derived from the same book the Catholics use, they just do things a little different. After all, just because they go to confession, say hail mary's, etc. doesn't make it heresy (same for the kooky snake-handlers and tongue speakers!)