"The term "Jewish Christian" is commonly used in the scholarly literature to denote early believers in Jesus who were not from a Gentile background but of Jewish stock (all of Jesus' twelve disciples, therefore, as well as Paul, were "Jewish Christians"). The term frequently has an additional nuance; it indicates those followers of Jesus who remained faithful to the Jewish law, the Torah; from this standpoint Jesus' brother James was a Jewish Christian but Paul was not. This latter group of Torah-observant Christians is sometimes described as consisting of "Christian Jews," rather than "Jewish Christians," in order to emphasize that their Jewishness remained as important or more important to them than their belief in Jesus' messiahship. 

As to the question of whether or not "Jewish Christian" is a justifiable term for contemporary Christians of Jewish background, such as myself — this is more complicated, partly because being Jewish is both a matter of ancestry and a matter of religious belief and practice. The question of whether Christians of Jewish background are Jewish is a subset of the larger and disputed "Who is a Jew" question, which is a matter of controversy both within and between the three main branches of Judaism and within the secular state of Israel. From the point of view of (orthodox) Jewish religious law, Jews who convert to Christianity remain Jews, albeit apostate ones. However, in the case of Brother Daniel, a Jewish convert and a Carmelite monk who applied for Israeli citizenship under the Law of Return, the Israeli Supreme Court refused to accept this definition because the past centuries of persecution of Jews by Christians meant that in most Jews' minds "Jew" and "Christian" had become opposites. 

My own religious practice is not Jewish; I do not, for example, try to keep the kosher laws or to observe the Sabbath. (Neither, however, do most Jews worldwide.) My religious identity for the past 30 years or so has been Christian. Culturally, however, my identity has strong Jewish elements. I read and speak Hebrew, identify with, while remaining critical of many aspects of, the state of Israel, and feel comfortable going to synagogue. I think of myself, therefore, as, in several important senses, Jewish, and I think that many of those who know me, both Jews and Christians, would agree. But I expect that many, probably most, Jews would disagree, especially when confronted with the abstract issue rather than with me as a person. For them, I am not a Jew; for myself, I am. Since "Jew" is not a term with a sharp definition but a social construction, it seems to me that there is room in the world for both views."

Joel Marcus is an author and Professor of New Testament and Christian Origins at Duke Divinity School.