It's hereditary or a very long conversion process (the latter of which is discouraged).

You are Jewish if your mother was Jewish - that's it. If your father was Jewish, you are not Jewish. Until recently, you could tell by DNA. My late father married a non-Jew, so I had to convert. My DNA is 50% Ashkenzi Jew. My daughter is Jewish because she was born to me, and, therefore, she is Jewish.

My brother and sister are not Jewish, nor is my mother. They did not convert. My culture is more Jewish, and so I took the decision to become Jewish.

Being a Jew has nothing to do with what you believe. There are a high number of Jewish atheists. Half of Israel is secular. 70% of Jews in Israel are people of color - only 30% are from Europe (Ashkenazi Jews). The majority of Jews in Israel have lived in the area for the past 2000 years (Middle East). They lived under Muslim rule, and prior to that, they lived under Christian rule. Both persecuted them.

When the Jews were given Israel by the United Nations in 1948, the Jews from the other Middle East countries were expelled, and they went to live in Israel.

Jews can have a Christian, Buddhist, Hindu religion - but if their mother is Jewish, then they are Jewish.