The religion of Judaism had been well established within the first millennium BC, and in Israel, prior to the rise of Christianity, Judaism remained the dominant religion of the people. Thus, Jesus was also Jewish, and all His earliest followers were Jewish. And yet, less than a century after Jesus was crucified in the early 30’s AD, the early Christian and bishop Ignatius of Antioch had written that “It is monstrous to talk of Jesus Christ and to practice Judaism” (Magnesians 10:3). So what happened?
The process of the divergence of Christianity from Judaism I think is best articulated by the renowned scholar Daniel Boyarin, in his book Border Lines: The Partition of Judaeo-Christianity. Boyarin, who is himself a Jew, reveals several important factors regarding how the two religions diverged, and as Boyarin shows, this must be done in the context of the study of the history of heresiology, or the history of heresy. Christianity and Judaism did not diverge as a result of the two views becoming more and more gradually dissimilar but as a result of the ‘leaders’ of each view defining the borders of their own worldviews over many centuries in order to exclude theological concepts they considered heretical, and these concepts usually belonged to the opposing view. The centerpiece theology of this debate was Logos theology, or the view that the one God was more than one person, so that, for the Christians, Jesus could also have been a part of the godhead, the view of binitarianism which stated that God was one being, but two persons. Of course, as Boyarin demonstrates at length, numerous Jews during the first century and earlier had already believed that God could have a multiplicity of the persons (as is reflected by the Wisdom traditions, Memra (Memra is the Hebrew word for ‘Logos’, and ‘Logos’ is the Greek word for ‘Word’, see John 1:1-18; “In the beginning was the Word, and the Word was with God, and the Word was God”) traditions, Second Temple exegesis indebted to Daniel 7 (‘Ancient of Days’ and ‘Son of Man’ discussions) and the discussion in the rabbinic texts regarding the idea of many Jews known as the ‘Two Powers in Heaven’.
Even though many Jews had held this view, by the time of the second century, the Rabbis began to view this idea as a heresy, as is first reflected in the Mishna (c. 200 AD) and then the Tosefta (c. 250 AD). From the first to the second century AD, however, the concept of heresy itself underwent a change in definition. The word heresy comes from the Greek haireseis, and by at least the time that the Book of Acts was written, this word only meant a choice of belief or adherence, belonging to a sect (of Judaism here, such as the other sects of Judaism from the first century like that of the Pharisees and Saudacees). This is clearly reflected in, for example, Acts 26:5, where we read “They have known for a long time, if they are willing to testify, that I have belonged to the strictest sect (haireseis) of our religion and lived as a Pharisee.” This passage undoubtedly reveals the early meaning of this phrase had no implications of what we would now identify with the concept of heresy — that is, a wrong, or contrary (heterodox rather than orthodox) belief. The change in definition came, at the latest, by the time of the writings of Justin Martyr c. 150 AD where Justin writes in his Dialogue with Trypho:
I will again relate words spoken by Moses, from which we can recognize without any question that He conversed with one different in numbers from Himself and possessed of reason. Now these are the words: And God said: Behold, Adam has become as one of Us, to know good and evil. Therefore by saying as one of Us He has indicated also number in those that were present together, two at least. For I cannot consider that assertion true which is affirmed by what you call an heretical (haireseis) party among you, and cannot be proved by the teachers of that heresy, that He was speaking to the angels, or that the human body was the work of angels. (Dialogue 62:2)
The pejorative use of the phrase “what you call”, combined with Justin’s other uses of the word haireseis, as Boyarin notes on pp. 40-41 of his monograph, demonstrate that by the time of Justin, the term ‘heresy’ had shifted from referring to a sectarian view to describing a wrong belief. A close cognate word to heresy was the Hebrew word minut, which was the term that the Rabbis after Justin used to describe a heretical view. Thus, now that there was a word and concept available from distinguishing between a simple viewpoint within a religion, to an incorrect and ungodly view of a religion, the leaders of Christianity and Judaism could define what constituted heresy and thus pave the borderlines around their ideologies that no one could cross, lest they reveal they were a heretic rather than a genuine believer.
Thus, in the second century, the Christians begun to claim that if you do not accept the multiplicity of God’s person (i.e. Logos theology), then you are not a Christian but a Jew, and the Jews said that if you do accept the multiplicity of God’s person, then you are not a Jew but a Christian. Thus, the borderlines of correct and orthodox Christianity had been paved, and the borderlines of correct and orthodox Judaism had been paved. There were some early ideologies, such as the Ebionites and Nazoreans, who claimed to be both Jewish and Christian. In other words, this was a hybrid worldview of the two religions. While these two views flourished most around c. 400 AD, it quickly becomes no surprise that Epiphanius (4th century) and Jerome (5th century) claimed that while they claimed to be hybrids, both Christians and Jews, they were actually “neither”. Thus, all middle ground was eliminated, and furthermore, the existence of these hybrid religions implied the existence of a pure version of the religion, not tainted by heresy (see pp. 207-210 and 212-4 in Boyarin’s book).
Other changes in definition also occurred during the 4th-5th centuries AD, as Boyarin also goes on to demonstrate. Religio (religion) for example, went from meaning ethnicities, populations and geographies, to referring to and characterizing belief systems. Superstitio (superstition) went from meaning excessive worship and obeisance to the divine, to simply becoming a word that refers to an actually incorrect practice or religion. Quickly, the available tools for the entire and complete divergence between Christianity and Judaism had started to become established. During the Second Temple Period, the Jews considered the world outside of Israel to be Gentiles. By the times of the composition of the final layers of the Palestinian and Babylonian Talmuds, the Christians and pagans themselves took on the role, in the Jewish perspective, of the Gentiles that had existed in the Second Temple Period. At the various councils of the 3rd-5th centuries, Christians began laying down their orthodoxies and correct beliefs, such as famously during the Council of Nicaea in c. 325 AD, whereas the Jews, inundated by the differing beliefs and interpretations, concluded that all views of the Rabbis come from God, and that in God’s understanding, all contradictions are resolved — and the only interpretations that could stop one from being a Jew was either Logos theology itself or the view that there can be one correct interpretation and that some Rabbis had gotten it wrong. This had concluded with the Jewish view that “an Israelite, even though he sins, is still an Israelite”, and thus defined the conceptual transition of Judaism from a religion to an ethnicity as is what we see today.
All this, collectively, explains and allows us to understand how the various sects of Judaism in the first century, including one so termed to be held by a small group known as ‘Christians’, paved the borders around what is acceptable Judaism in a way that starkly contrasted it with the world of the Christians who, themselves, began paving the borders around their own acceptable beliefs as they begun to rise and conquer the Roman Empire. The centrality of this division was Logos theology, the view that God could have more than one person alongside the Father (and that would be Jesus for the Christians, this is otherwise known as binitarianism), which, although was a common Jewish belief before Christianity, become unacceptable during the 2nd century. The rabbis tried to equate binitarianism with ditheism (the belief that there are two gods) and condemned the entire concept as a heresy, or minut. This is how, as Boyarin shows, Christianity and Judaism had diverged. Boyarin’s case goes much more in depth than this, and I cannot do it justice here, analyzing all sectors and developments of the evidence, and is a highly recommended read and is certainly one of the most influential scholarly works since the beginning of the 21st century. Alas!