The no true Scottsman fallacy
Evolutionist: All scientists believe in evolution.
Creationist: What about Michael Behe?
Evolutionist: He's not a scientist.
Creationist: Why do you say that?
Evolutionist: Because he doesn't believe in evolution. All true scientists believe in evolution.
No true scientist would ever deny evolution.
Orthodox Jew: All Jews deny that Jesus is the messiah.
Messianic Jew: Wait a minute. I'm a Jew, and I think Jesus is the messiah.
Orthodox Jew: You're not a Jew.
Messianic Jew: Why do you say that?
Orthodox Jew: Because you think Jesus is the messiah. That makes you a Christian, not a Jew.
Messianic Jew: I am a Christian, but I'm also a Jew. "Christianity" is a subcategory within Judaism.
Orthodox Jew: Now who do you think is in a better position to say what a Jew is? A Christian or a Jew? As a Christian, you have no right to say what constitute a Jew. Only Jews have that right.
Messianic Jew: But I am a Jew, which gives me the right to say what constitutes a Jew.
Orthodox Jew: No, you're not. You're a Christian. Christians are not Jews.
Round and round they go.