Actually, it's not as simple as that, because the words "Christian" and "Jew" have very different meanings today than they did in antiquity.
In fact, Christians were Jews, albeit a specific kind. However, there have always been different kinds of Judaism; or, to put it conversely, there has never been one way to be Jewish.
Indeed, there are even atheist Jews, and not just secular ones -- just google atheist rabbi. I'm not sure how they manage that, but in practical terms, the majority of (ethnic) Jews can't be (religious) Jews, or they wouldn't support the Democratic Party. The majority of seriously religious Jews naturally tend to be conservative, and are aware of the fact that liberalism has become a substitute religion for their irreligious fellows. Which only violates the first two Commandments. Not to mention the the sixth through tenth.
In addition to liberalism, the other thing that unites secular Jews is their anti-Christian attitude. Given their traumatic history (albeit in Europe, not here), it frankly isn't difficult to understand this, for the same reason it isn't hard to understand why blacks would despise the party of slavery, Jim Crow and segregation, i.e., the Democrats. Oh, wait...
Actually, the cases aren't that dissimilar, for just as a minority of blacks understand that conservatives are their greatest friends, a minority of Jews understand that Christians are their most staunch and devoted allies. Conversely, virtually all of the wholesale anti-Semitism in the world comes from the international left (and from Islam, of course).
Thus, although Boyarin seems to think that his findings will be equally unsettling to Christian and Jew, relatively few Christians will be disturbed to learn they are even more Jewish than they had realized, whereas the only thing many secular Jews know about their religion is that it is not Christianity.
But the opposition between the two only occurred over time. Instead of being two types of Judaism -- i.e., bound by their commonality -- they eventually began to define themselves by their differences. It's analogous to a bunch of chess pieces initially defining themselves as pawns, knights, rooks, et al, but then deciding to define themselves as black or white. The pieces haven't changed, only the self-identification.
Me? I love the idea that what Christians consider unique about the Christian revelation actually has deep roots in Jewish scripture, most controversially, trinity and incarnation. I guess Jews are supposed to get all farklemt or farmisht about these commonalities, but it's right there in their scripture.
Boyarin goes straight to Daniel -- coincidentally (?) the last book of the OT in the Orthodox Study Bible -- where we read of (what else to call it?) two Gods, the Ancient of Days and the Son of Man. The latter is an odd designation, but it is precisely the one Jesus most often applies to himself.
Daniel 7:9 describes a second divine throne, and in 7:14 it speaks of how the Ancient of Days transfers to the Son of Man "dominion, honor, and the kingdom." "His authority is an everlasting authority" and "his kingdom shall not be destroyed."
So there's that interesting little item. I've also always been intrigued by Genesis, where God is quoted as saying "let Us make man in Our image, according to Our likeness"; and to those passages of Proverbs which speak of the eternal pre-existence of wisdom, e.g., 8:27-30, "When He prepared the heavens, I was there, When He drew a circle on the face of the deep.... I was beside him as a master craftsman."
For Boyarin, it is not possible to regard Jesus as some sort of aberration from the mainstream -- or at least one of the main streams -- of Judaism. For example, "many Israelites at the time of Jesus were expecting a Messiah who would be divine and come to earth in the form of a human." Thus, it is no longer possible "to think of some ethical religious teacher who was later promoted to divinity under the influence of alien Greek notions...."
During the first few centuries of "Christianity," there were many people who were unproblematically both "Jewish" and "Christian." However, they would have identified themselves as simply Jewish. That is, they continued to follow Jewish dietary law but also believed in Christ as son of God.
In point of fact, the definitive break didn't come until the fourth century, when Constantine called for the first ecumenical council in order to clarify just what Christianity is. Thus, oddly enough, you could say that the Council of Nicaea simultaneously created both Christians and Jews, for the Council emerged with "the establishment of a Christianity that was completely separated from Judaism."
But before this, "no one... had the authority to tell folks that they were not Jewish or Christian, and many had chosen to be both." Only afterwords were these Christian Jews or Jewish Christians "written right out of Christianity."
It reminds me a little bit of how I am the same liberal I've always been, except that the left has now written classical liberals out of their script(ure). If you're not a leftist, you're somehow illiberal.
Back to the Son of Man business. I've only just started the book, but again, Jesus most often refers to himself by this title, so what does it mean?
Interestingly, Boyarin suggests that we have things backwards -- that Son of Man is a divine title, whereas Son of God is a human one. To support this thesis, he points out that "Son of God" is all through the OT, referring to how earthly Kings such as David were ritually anointed with oil and became "sons of God."
But what could Son of Man refer to? I have always considered it to mean something like Mankind v2.0. In other words, if you believe that humans are descended from apes, you could in a sense say that human beings are Sons of Apes. Analogously, Jesus represents another evolutionary leap, making him the Son of Man.
Boyarin suggests something similar, as if Adam is indeed mankind v1.0 and Jesus is mankind The Sequel. But that's about as far as I've gotten in the book. I'm up to page 40, where Boyarin notes that the two divinities referenced above -- the Ancient of Days and the Son of Man -- "in the course of time, would end up being the first two persons of the Trinity."
So it seems that Christian theology may not be quite as meshuge as many Jews believe. And that those anti-Semitic liberal Christian denominations need to stop boycotting themselves, i.e., Israel.