One of the issues that rises to the top of any discussion of the definition is whether the terms “Jewish Christianity” (Juden-christentum) or “Jewish Christian” (Judenchrist, Judéo-chrétien) refer to the extraordinary influence of Judaism, theology and practice, on early Christianity or to an actual segment of believers in Jesus who were in someway “Jewish” and thereby distinct—the distinction most often understood in nationalistic and Torah-oriented ways—from other believers in Jesus. The terms, then, can be used on the one hand to characterize what the two religions have in common and, on the other, point to what divides them. In the history of research, the terms have been used variously to refer to either one of these concepts and therefore are ambiguous and susceptible to very different usages.
Of equal importance to the definition of such terms when speaking of a segment of early Christianity is the question of whether one could speak of a unified group with a definable profile of traits and ideology/theology or whether in fact Jewish Christianity was a variegated phenomenon, in which case a plurality of Jewish Christianities would be a more accurate understanding. One final point of contention with respect to the definition of terms is whether one should use a praxis-, ethnic- , or ideology/theologic-based criteria for determining whether a particular writer, text or group is Jewish Christian.
O. Skarsaune in the first chapter of Jewish Believers in Jesus seeks to address these difficult issues of definition. The first step he takes is to avoid to some degree the baggage of the history of research by using the less loaded phrase “Jewish believer in Jesus”. This is not without historical precedent as well. While there is no fixed terminology in patristic sources, the phrase “Jewish believer in Jesus”, according to Skarsaune, “can be said to encapsulate the terms most often used”. He provides a broad series of examples from New Testament and early Christian sources of the use of terminology similar to “Jewish believer (in Jesus)”. For example:
John 8:31 - “Jews who believed in him . . .”
Origen, Cels. 2.1 - “. . . those of the Jewish people who have believed in Jesus”
Origen Comm. Matt., in Eusebuis, Hist. eccl. 6.25.4 - “[Matthew published his gospel first] for those who from Judaism came to believe
Furthermore, Skarsaune defines “Jewish believers in Jesus” as “Jews by birth or conversion who in one way or another believed that Jesus was their savior”. With this definition the issues noted above are addressed at least to some degree.
First, Skarsaune opts for an ethnically based definition rather than praxis or theologically based one. While the editors of the volume are not the first to base their definition primarily on ethnicity this approach to the question is less than typical in secondary literature. Thus, under the rubric of Jewish believers in Jesus they include not only Jews who after conversion maintained a law-observant lifestyle, but also Jewish converts who abandoned their Jewish identity and assimilated into Gentile Christianity. This definition, however, excludes Judaizers, by which he means Gentiles who after conversion to belief in Jesus underwent a secondary conversion to Judaism by circumcision and the taking on of Torah observance. Skarsaune, then, uses the moniker “Jewish believer in Jesus” to represent all ethnically Jewish believers regardless of their continuing relationship (or lack there of) to Torah, while reserving the traditional label “Jewish Christian” for “those Jews who believed in Jesus, and at the same time continued a wholly Jewish way of life”. The definition of "Jewish Christian" offered would also include Gentiles who prior to their faith in Jesus converted to Judaism and subsequently to belief in Jesus.
 Carleton Paget 2006:48; see also his earlier essay Carleton-Paget 1999. One of the most interesting points in this well-researched chapter is that the real birth place of the study of Jewish Christianity is England in the works of Toland and Morgan in the 18th century rather than Germany in the work of Baur in the 19th as is most often thought. It is not surprising that Carleton Paget, the quintessential Englishman, would have made this discovery.
 I am unclear what exactly Carleton Paget thinks. In reading between the lines of his conclusion, I gather that he is more of a mind to abandon the terms altogether for something else. This seems to be the point of his discussion of Michael Williams’ work on Gnosticism—see Carleton Paget 2006:51-52.
 See the summary of M. Simon’s position by Carleton Paget (2006:43).
 Skarsaune 2006a:5.
 Examples come from Skarsaune 2006a:5-6.
 Skarsaune 2006a:3.
 Skarsaune 2006a:4, emphasis his.
_________. 2006. The Definition of the Terms Jewish Christian and Jewish Christianity in the History of Research. In A History of Jewish Believers in Jesus: The First Five Centuries, ed. Oskar Skarsaune and Reidar Hvalvik:22-52. Peabody, MA: Hendrickson.