Judaism is unlike stuff like Christianity and Islam in that it has never been about spreading itself and conversions.
It has basically always been a communal religion, the religion of a specific cultural group of people.
So as a result there is the Jewish religion, and there is being culturally/ethnically Jewish. You can of course be born and raised in Jewish culture while also not being religious, be an atheist even. Presto, you would be a secular Jew.
Still a smart boi I see