Jews and Christians are two separate belief systems. Christianity used to be close to Judaism when it first was created, but now there are many differences in beliefs.
A Christian is a Christian regardless of where they reside. A Christian is never a Jew and a Jew is never a Christian. The two belief systems are incompatible.
Bonus Question. I0 or incredibly desperate. When they suggest that "rabbinical Judaism" somehow isn't valid but their version is, it simply doesn't get more ridiculous than that.