Weren't you the one who mentioned the genetic part of Jewishness??? My friends are Jewish by race and Christian in their faith. You are not correct in defining Messianic Jews as modern-day Pharisees who are looking for the Messiah to come one day. They are people who were born Jewish (race and religion) and have converted to Chrisianity from Judaism. They believe that Jesus Christ (Yeshua Hamashiach) is the true Messiah. There are several on FR that post from time to time. Ask them.
As far as a Muslim becoming a Jew or Christian, of course they couldn't because Muslim is a Religion not a race. Can an Arab be Muslim or Jewish or Christian? Yes. See the diff?
Sure. There are Jews who are actually racially Semitic, i.e. genetically related to the Hebrews, and all other Semitic peoples. But, according to the Jewish law, you are a Jew if your mother is Jewish. The genes don't matter, be they father's or mother's! It's the mother's "Jewishness" that matters.
The mother, for all you know, could be a convert to Judaism who could be genetically completely unrelated to anything Semitic. The "Jewishness" of the mother (whatever the criteria) is somehow indelibly "imparted" to her children so they are all born "Jewish."
Is being "Jewish" an ethnic or religious or national issue? I guess, it depends what suits you (or what will give you a visa out of the country you don't want t live in).
According to the Society of Humanistic Judaism" [all emphasis are mine] "There is no single way to be Jewish..." [surprise!]. The Society affirms that "Secular Humanistic Jews encourage and support activities that promote the continued development of Jewish identity." (which by their own admission is an elusive concept). However, it all basically boils down to "a deep attachment to the state of Israel, its culture, and its people."
Furthermore, the source states "Secular Humanistic Jews make no distinction of any kind among Jews who, regardless of parentage, have chosen to identify with the Jewish people."
Yet being a Muslim or a Christian prevents you from being considered Jewish in Israel or by the Jewish Community at large. The Israeli Law of Return is a law that grants immigration rights only to the Jews without a prior naturalization process; that is – to those individuals who "fit" the official (rabbinical) definition of "Jewishness." The Law simply states that "any Jew may come to Israel and become a citizen without undergoing naturalization."
Specifically, zionism-israel.com's Ami Isseroff explains that "Israel's immigration policy is not racist or separatist. In common with many other countries, it gives precedence to its own absent nationals."
So, Israel treats Jewishness as an inherent nationality (that is a matter of choice), independent of where you were born or how many generations have passed. Others treat it as a religious affiliation, and others yet as a racial/ethnic quality. [covering all the bases, I guess...]
As for Messianic Jews, they are a periphery, embraced only by such Christian groups as the Presbyterian Church [sic] (U.S.A.) which for a long time was in "communion" with Avodat Yisrael. Messianic "Jews" are Protestants in disguise with Jewish terminology and Jewish-sounding names. Their aim, of course is to "save" as many Jews by converting them to Jeezuhs.
Jasom Baysee, a pastor of Shady Grove United Methodist Church in Providence, North Carolina, writes in The Christian Century "If there is anything about which all four branches of Judaism in the U.S. (Orthodox, Conservative, Reform and Reconstructionist) agree, it is that one cannot be both Jewish and Christian at the same time."
If you are going to be a religious (observant) Jew, you cannot worship any other God except the YHWH of the Old Testament. The Jewish messiah is not a divine person. That automatically disqualifies Jesus as a possible Jewish messiah. The idea that the messiah is a divine person is a Christian innovation that was rejected, along with all Christian books, by the rabbis in Jamnia at the end of the first century AD.
Messianic (Apocalyptic) Jews of the 2nd century BC (Pharisees and Essenes) expected a messiah to be a mortal warrior-king (there are 7 OT determined requirements for a messiah and Jesus meets only one, being Jewish). As such a messiah for the Jews is not an incarnate God-man who, together with the Father and the Spirit is worshipped and glorified. That is heresy in Judaism across all sects.
So, they cannot deny that someone "born" Jewish is Jewish by Law, but if they worship any other deity (and no, the Jews do not think Christians and Muslims worship the same God as the Jews, because then they would have to admit that Christ is God and Allah is God, which they won't!), they are no longer Jewish, but apostates no different than all the other Jews who worshipped false gods in the past.