As I’ve written before somewhere in this voluminous blog, the definition of “Jew” is complex because Jews are both a religion and an ethnic group. They are also a people, which is somewhat harder to define.
Often there’s overlap, but often there’s not. For example, you can be a secular ethnic Jew who doesn’t practice Judaism at all – or even is against it – and much of the world will still consider you Jewish. You can also be a convert to Judaism and not ethnically related at all, but you automatically become Jewish on conversion. Your DNA may not show it, but you’re a Jew by Judaism’s definition, and you are also considered part of the Jewish people.
So an ethnic Jew who’s an atheist remains an ethnic Jew. And an ethnic Jew who converts to Christianity, or embraces what’s called “Messianic Judaism” by its practitioners, remains an ethnic Jew as well. He or she may consider himself or herself to be a religious Jew as well, but many religious Jews (not all) would beg to differ. Judaism excludes belief in the divinity of Jesus. A basic tenet is that the deity is unknowable and unnameable and unpersoned. Once you believe in the divinity of Jesus you’re not practicing Judaism even if you think you are, but you remain an ethnic Jew. Judaism differs from Christianity in many ways, but this is one of the most basic.
But what, you might ask, about Jewish belief in the Messiah? It’s quite different from the Christian belief. The Jewish messiah is a mortal person, a “a fully human non-deity Jewish leader, physically descended via a human genetic father of an unbroken paternal Davidic line through King David and King Solomon.” References to being the “Son of God” are metaphoric rather than literal. Nor is belief in a Messiah of any type universally true even of religious, believing Jews.
The definition of being a Jew is complicated somewhat by the fact that different strains of Judaism believe that being ethnically Jewish – or part of the Jewish people – is inherited in different ways. The more Orthodox believe that Jewishness involves having a Jewish mother and even if the father is not Jewish the offspring are Jewish (part of the Jewish people) as long as the mother is Jewish. Reform Jews believe Jewishness is passed through either parent.
The legal system of Israel reflects some of this, but Jews outside of Israel aren’t bound by it in any way unless they happen to agree with it anyway. All Jewish denominations around the world reject Messianic Judaism as a form of Judaism in the religious sense. However, Messianic Jews can become Israelis under the right of return – but only if they qualify in terms of ancestry.
There were several Israeli court rulings that are relevant to this; they concern the narrow issue of the right to return. Earlier law had allowed ethnic Jews to return unless they converted to another religion (atheists were okay). But a newer ruling allowed Messianic Jews to return if they qualified as ethnic Jews, but only if the inheritance was through the paternal line and not the maternal line. To me, it seems that this somewhat mixes up the definitions of what is a Jew and blurs distinctions between the religious definition, the legal one for the right of return, and the ethnic one. After all, ethnically, someone with only a Jewish father who then converts to Messianic Judaism is no different from someone with only a Jewish mother who then converts to Messianic Judaism. But the law in Israel treats them differently. Anyway, here’s a description of the case:
The state of Israel grants Aliyah (right of return) and citizenship to Jews, and to those with Jewish parents or grandparents who are not considered Jews according to halakha [traditional Jewish law], such as people who have a Jewish father but a non-Jewish mother. The old law had excluded any “person who has been a Jew and has voluntarily changed his religion”, and an Israeli Supreme Court decision in 1989 had ruled that Messianic Judaism constituted another religion. However, on April 16, 2008, the Supreme Court of Israel ruled in a case brought by a number of Messianic Jews with Jewish fathers and grandfathers. Their applications for Aliyah had been rejected on the grounds that they were Messianic Jews. The argument was made by the applicants that they had never been Jews according to halakha, and were not therefore excluded by the conversion clause. This argument was upheld in the ruling.
I hope this post clears up some questions – although I have my doubts.