Judaism is both an ethnicity and a religion. Even your friends agree with that.
My friends? Huh? You're saying odd things.
As an ethnicity, could it get much thinner? It's an ethnicity, because of religious culture more than genetics. Just one parent, and you're Jewish? How does that work out, after 3 or 4 generations?
Also, apparently, I become a part of this ethnic group, by converting to Judaism.
Korea is a Korean State. Japan is a Japanese State,
Can I become an ethnic Japanese, by believing things in an iron aged book? I don't think so.
Stop playing dumb.