Being Jewish is both an ethnicity and a religion. Being born of a Jewish mother makes you Jewish, but if a woman joins another religion, and then has a child, that child is not Jewish.
When you convert to Judiasm, by custom, you are considered Jewish. If you are a woman, and you have a child, that child is considered Jewish. ... if you converted before having that child. It might not make you 'ethnically' Jewish, but that is not relevant on how you are considered under Jewish law.
If a woman is Jewish heritiage, but practicing another religion, her children are not considered Jewish
If Jews practice another religion, they might be considered ethnically jewish, but not of the Jewish faith. If they converted, they can come back without going through a conversion process. Any children they had while practicing the other religion are not 'Jewish' under the law, and would have to go through the conversion process. OF course, that would involve worship of God. If there is no God involved in the other religion (such as many types of Buddhism), that does not violate the 10 commandments, 'Tho shall not have any God before me'. Therefore, someone who went straight to atheism, and did not practice another religion could still be Jewish. For that matter, there are many atheists that are quite religious Jews. It gets bogged down and confusing to many Christians.
The term JUBU has been used to reference people who are Jewish Buddhist. There are some that follow both traditions at the same time... confusing as all heck. They are not a majority though.
However, if you practice another religion that concerns itself with God, then, you are no longer considered of the Jewish faith.