Why do fundamentalists choose the times when Catholics are Christians and when they're not?

Many fundamentalist Christians don't like Catholicism, and they often make the claim that Catholics aren't even Christians, which doesn't make sense since Catholics were the earliest Christians. But when it's convenient for them, they will take an event like the Crusades that makes Christians look bad and say it was the Catholics (who were the only Christians at the time). So Catholics seem to become Christians to them only when they want them to look bad. Is this fair?