Religions are just ideas, theories, and morals, backed up by allegorical fairy tales. The only way you could justify calling someone a Jew is if they strictly follow all the rules and guidelines of their religion. And when I say "strictly" I mean without any slight imperfection. Otherwise, they're just a human being. Same goes for Christians and Muslims. There are people who call themselves Christians but at the same time have pre-marital sex, swear, steal, lie, cheat, and rarely attend church. What makes you a "Christian" then?

I genuinely do not understand religion, or its purpose.