That's pretty much the exact point I've been making. Christianity is NOT an ethnicity. Christian is conceived as a universal religion. Judaism is not and was not. Jews are "the chosen people" and they have always conceived of themselves as separate and different. The difference is that the term Jewish acts as a proxy for both an ethnicity and as a religion. That's completely distinct from Christianity or Islam. Your example of Catholic/Iberian is very apt.

Sure, Jews can cease practicing Judaism. They can convert to Christianity or whatever. One of my childhood best friend's mom is a convert and actually became a pastor. I think she was slightly active with Jews for Jesus, but that's getting far afield. Her children who are neither Jewish by upbringing nor Jewish by religion nor Jewish by culture are not Jewish. They no longer have that ethnic connection.