Rabbinic discourse about Two Powers in Heaven is not a rabbinic “report” of essential differences between Christianity (or “Gnosticism”) and Judaism, but rather a rabbinic production of the defining limits of what the Rabbis take to be Judaism via the abjection of one traditional element in Jewish religiosity, and production almost identical, as we shall see, to the Christian heresiological naming of One Power in Heaven (Monarchianism) as “Judaism,” when, in fact, it was, of course, an internal and once-acceptable version of Christian theology. I am suggesting that for the Rabbis, the discourse of heresiology, that is, the collection of laws and narratives about minut and especially about the “heresy” of Two Powers in Heaven, is not about Christianity but may be in part a response to Christianity. (Boyarin, Border Lines, 133)
I would like to query Boyarin's assertion that "One Power in Heaven" (a reformulation of Monarchianism from an explicitly rabbinic perspective) was "an internal and once-acceptable version of Christian theology" (my emphasis). Boyarin has presented compelling evidence that binitarianism/Logos theology/Two Powers in Heaven was a Jewish theoloumenon — and a relatively common one — from before Jesus even until the Middle Ages. Here not only Philo's Logos theology (as well as his surprising reference to the Logos as a deuteros theos [cf. Questions on Genesis 2.62]!) but also the Memra theology found in the Targummim (and effaced in rabbinic literature, including the "rabbinized" Targum Onkelos and Pseudo-Jonathan) suggests that the rabbinic program of (and Christian collusion in) denying "Two Powers" theology to "Judaism" were discursive rather than descriptive.
But it is precisely this compelling marshalling of evidence that makes me wonder, On what basis can we claim that Monarchianism was was a "once-acceptable version" of Christian theological expression? I'm not suggesting there isn't such a basis; I'm simply unaware of what that basis is (and a bit perplexed that Boyarin asserts this point without anything like the compelling demonstration of his point vis-á-vis the Judaic heritage of binitarianism). If any of you are aware of such evidence, I would greatly appreciate hearing it. On the other hand, I would also caution that certain evidence that has been read for an early Christian rejection of Jesus' unique (or at least unusual) status vis-á-vis Israel's God is not compelling (e.g., that Mark 10.18 intentionally distances Jesus from God; cf. Mark 10.21, and especially Jesus' instruction for the rich man to follow him).
My suspicion is this: That the anathematization of "One Power in Heaven," as Boyarin would likely agree, can indeed be read as evidence of continued Christian-Jewish interaction and even inter-identification (given that "Christian" and "Jewish" were not yet descriptors that applied to concrete "things" or groups). But I doubt that the naming of "One Power" theology as Christian heresy is the discursive mirror of the naming of "Two Powers" theology as Jewish heresy. In other words, the declaration that "Two Powers in Heaven" was heretical by would-be Jewish heresiologists projected a previously kosher form of Jewish theology outside the acceptable boundaries of Judaism (again, a discursive rather than descriptive act). But the discursively similar act by would-be Christian heresiologists, by which they declared Monarchianism heretical, was not necessarily the projection of a previously sanctioned form of Christian theology beyond the pale.