If Harvey Wienstien (a jew through 13 years etc) marries a Christian and goes to christian church every Sunday, is he just as much a Jew as Barrack is a muslim???
Actually, yes. Jews and Muslims are Biblical cousins, this relationship can be traced through Abraham, Isaac, Jacob, and Ishmael, where Jews regard Isaac as the father of their people and Ishmael as the father of the Arab people, but both Isaac and Ishmael were born from Abraham.
Simply put, being Jewish or Muslim is considered much more of an ethnicity than a religion. You're born into it in other words. That's why Muslim leaders around the world will always view Obama as a Muslim, as he was born to a Muslim father. In Islam, that's enough to be considered Muslim, where Jews differ in this regard...
Now, if this Harvey Wienstien guy (a Jew) that you're speaking about marries a Christian woman, according to Jewish law their kids will be considered Christian, but Harvey will remain a Jew, regardless if he attends Church with his family.
According to conservative and orthodox Jewish law, if your mom is a Jew and your father is a Gentile, you're still a Jew. But if your dad is a Jew and your mother is a Gentile, you're a Gentile. Bottom line, you can NEVER be a Jew if your mom isn't a Jew.
Reform Jews add one more possibility of being Jewish however... They say that you can still be considered Jewish if your dad is a Jew and your mom isn't, just as long as you were "raised Jewish", i.e. Bar Mitzvah'd, etc. Of course this isn't Biblically correct and Jewish scholars have been fighting against this position of Reformers' for a long time.
But conservative, orthodox, and reform Jews all agree that having a Jewish mother makes you Jewish, period. The father can be an atheist, Christian, Muslim, etc. and that wouldn't affect the child. This is in accordance with the Torah.