There's a difference between "Christianity" and "the Church", and that nuance is important. "Christianity" (in some languages, "Christianism") is more of an umbrella term for the religion--it functions more as a descriptive for a "school of thought". People can be Christians, but they are not said to have "joined Christianity", they join a Church. The Church is called one, holy, catholic, and apostolic, but it is not called "Christianity".
I believe that Roman Catholics and Protestants are Christians, they are a part of "Christianity". Are they members of the Church? In a basic sense, since they are not Orthodox, no they are not. Almost certainly Protestants reject enough of the faith to be solidly outside the Church, IMO. What about Roman Catholics? What we differ on is, in one sense, very little, but in another sense so important that it might as well be a lot, so again, IMO I don't know. But "I don't know" is still a really bad place to be.