Being Jewish Versus Doing Jewish
First, let us review Jewish law. Most Jewish scholars agree that you cannot convert away from Judaism (there are a minority who say you can convert away). However, this by no means implies that a Jew who joins another religion has the same standing as a Jew who has remained Jewish. A Jew who practices another religion, whether it be Christianity (in any of its forms, including Messianic "Judaism"), Islam or anything else, is known as an "apostate." An apostate is someone who has removed themselves from the Jewish people by joining another faith. The apostate cannot be buried in a Jewish cemetery. Nor can the apostate be counted for a minyan, the minimum of ten adult Jews (or adult Jewish men, depending on one's denomination) needed for saying certain prayers. For nearly all practical purposes, they have the same status in the Jewish community as non-Jews. The only differences is that they do not have to undergo a full conversion to rejoin the Jewish community, though there is a process known as "Teshuvah" that they must go through to come back to the Jewish community with full Jewish status. So, once we delve into Jewish law we see that a Jew who joins another religion, though still Jewish, has ostenisbly the status of a non-Jew and is no longer able to participate as a full member of the Jewish community. The apostate has the weakest of all graps to their Jewish identity, their are Jewish by birth but otherwise have no position in the Jewish community.
This then brings us to a discussion on the difference between "doing Jewish" and "being Jewish." By "doing Jewish", I am referring to someone who is actually practicing Jewish beliefs and laws. By "being Jewish", I mean someone who has been born Jewish. Now, there are many people who are born Jewish but are not practicing Judaism. For instance, if a Jewish person murders a family, the Jewish person may be "born Jewish" but certainly was not "doing Jewish" as murder is a violation of the Ten Commandments. Similarly, we can see examples of this in the Jewish Bible. The prime example of this would be the Jews who worshipped the Golden Calf at Mount Sinai. These individuals were unquestionably Jewish by their birth, thus we can argue that they were "being Jewish." However, were they "doing Jewish?" The Bible is quite clear that these individuals, though born Jewish, were not practicing Judaism. The worship of the Golden Calf, though it was a monotheistic form of worship, was clearly a violation of the Biblical law regarding the making of idols. Thus, they were not "doing Jewish." Similarly, in the Prophets we see countless reprimands of people who were undeniably Jewish by birth, but had entered into apostasy by worshipping gods such as Baal and Ashtoret. They were not "doing Jewish" but "doing pagan" and had violated the very underpinning of Judaism. That they were "being Jewish" by their birth was clearly not sufficient. One must also "do Jewish" in their actions. Along the same lines, many of Jesus' original followers were unquestionably born Jewish. However, by adopting non-Jewish beliefs, such as thinking that Jesus was a god (a violation of several places in the Jewish Bible, Numbers 23:19 and Hoshea 11:9 provide a few examples of this), were no longer "doing Jewish." The same is true for individuals in the Messianic movement today. The Messianic movement was created by Christianity, and its beliefs and values reflect Christianity. By practicing Messianic "Judaism", the individual who was "born Jewish" is no longer "doing Jewish." They have removed themselves from the Jewish community by their practice and beliefs. Unfortunately, too few of these individuals have an understanding of the diffierence between "being Jewish" and "doing Jewish", mistakingly believing that they can both believe in Jesus and still have the same status in Jewish community as before. They not only have lost status, but also have lost the practice of Judaism. As our wise sages pointed out in Pirkei Avot, the emphasis is on the action, the "doing Jewish."