"Jewish" is technically like two words which refer to two separate things, but use the exact same spelling and pronunciation. In the ethnic sense, Jewish refers to semite/hebrew culture. In the religious sense it refers to followers of the different interpretations of the oldest of the Abrahamic faiths.

This is how you get a lot of atheists who still call themselves, or ar considered jews. On an interesting side note, it's also how certain christians in the middle ages could still be thought of as jewish. There's a scene from a biopic about Michelle De Nostradamme (a.k.a. Nostradamus) that has an early scene where his grandfather tells him that he's jewish. The kid is shocked by this "but grandfather, I've been baptized". His gramps responds "ture enough, but in the eyes of the inquisition, a baptized jew is still a jew."