There is no such thing as an ethnic Jewish past. The whole notion of an 'ethnic Jew' was coined in the late 19th century, among bigoted 'scholars' in Austria and Germany. At its core, it is little more than xenophobic political pseudophilosophies dressed up as anthropological and genetic truths. Prior to this time, the Christian Church considered Jews to be a religious group, as did the Jews themselves. They treated us as a religious group, and anti-Jewish violence perpetrated by the larger community had a clear religious slant. The expulsion of the Jews from the Iberian Peninsula, or from England, took the form of one larger religious community driving out another. Religious clerics pressured the Jews prior to their departure from those countries to become Christians, and so avoid persecution in the present. The same is true for the Spanish Inquisitions---and for the earlier Central/Western Euro Inquisitons that led up to it...those former Jews (conversos) who were charged as 'heretics' by the Spanish community were so charged because they were believed to STILL be Jewish (that is, they were charged for continuing to observe Jewish religious practices).
In the late 19th century Germany & Austria, things changed. The Jews became secular, and more involved in the non-Jewish community. The bigots felt threatened, but realized that they had lost their excuse for hating Jews (religion). In search of another excuse for his antiJewish bigotry, a man named Wilhelm Marr decided to impose his sick worldview on linguistics. He resorted to finding a meaning in two groups of languages that were never supposed to be there; namely, the "Semitic" group of languages, of which Hebrew is one, is evidence for a 'Semitic' race, and the 'Aryan' group of languages (which we now call the 'Indo-European' branch, since the term 'Aryan' has baggage) is evidence of an "Aryan" race. This only makes sense if you also believe that the hundreds of Papua New Guinean communities that have remained isolated from the world for centuries and are now shifting to speaking a Germanic language (Tok Pisin) are ethnically Germans. Obviously, the "Semitic" race was a threat to the "Aryan" race...plotting a political takeover, going to outpopulate the 'Aryans" and dillute the gene pool, exc. You can see where this line of 'thought' led. Hitler loved it. So do Messis.
There are Ethiopian Jewish communities, Ugandan Jewish communities, Indian Jewish communities, Sephardic Jewish communities, Ashkenazi Jewish communities, Arab Jewish communities, Persian Jewish, and so many other Jewish communities of ethnically diverse backgrounds. To claim that there is something like 'Jewish ethnicity' is to slap these communities--in addition to the ones who suffered directly from the poisonous political propaganda disseminated by Marr and, later, Nazis-- in the face. And that would be un-Christian...at least according to my formerly Jewish, Catholic mother.
There are also plenty of Christian clergy who find the notion of using Nazi ideology to decieve Jews into conversion to Christianity to be deplorable and un-Christian behavior, including the Evangelical Lutheran Church, and the Presbyterian Church.
Peace
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Halakhakly, children born to a Jewish mother are considered Jewish, not because the mother has special blood that she passes to her child, but because the Jewish soul is passed from mother to child. To say that one inherits "Jewishness" from the mother because 'it's in the blood' is an obvious indulgence in the Nazi ideology on which the "Messianic Jewish" concept is grounded. Apostate Jews can betray their Jewish souls, and are thus shunned from the Jewish community. There are many passages in the Tanakh that support this.