I understand that, but if someone says they are “Jewish by birth and nonbeliever by belief,” I would think they would be in the ethnic group but not the religion, as opposed to someone who does also practice the religion.
That makes sense, personally I would consider nonbeliever to be only people who don’t follow any religion, but I can see how it is more up to interpretation.
It wasn’t until my late 20s that I learned how “relaxed” Judaism could be. I hadn’t thought you could be part of a religion yet a nonbeliever at the same time.