How does believing in Judaism make you a Jew?

No offense, but I am just curious.

Didn’t the term ‘Jew’ originally refer to the people who lived in Judea a long time ago?

Wasn’t it originally an ethnic term?

However, it seems that the term ‘Jew’ is not solely a religious term either.

People whose parents or grandparents believed in Judaism are also considered Jews, even if they do not practice Judaism.

But having Christian parents does not make you a Christian if you are not a Christian.

It seems to me that the term ‘Jew’ used to be primarily an ethnic term, but over time it has also acquired religious connotations.

However, it does not make sense that you can become part of an ethnic group merely by adopting the religion practiced by many people in that group.

You would not be Hindi even if you believed in Hinduism.

You would not be Japanese even if you believed in Shinto.

But somehow, you are considered a Jew if you believe in Judaism.