the people in the Synagogue were furious (Luke 4:28, NLT)
I want to be delicate here in nuancing this argument about the most likely audience of James' epistle. First, I will make three points about Luke's story, and then I'll get to the epistle.
Point One: That Luke's Nazarines are "filled with wrath" (ESV) is an aspect of Luke's narrative portrayal. Whether or not a group of people from the synagogue gathering actually attempted to throw Jesus off of a cliff is not a question I'm asking right now. That question matters for other discussions, but not for this one. In this post, I am only taking Luke 4:25-30 as an artistic depiction rendered by Luke and presented to his audience. For our purposes today, that story may or may not have been based in actual fact.
Point Two: Modern scholarship has rightly and justifiably objected to any interpretation of this passage which supposes that animosity towards gentiles was typical among Jews anywhere in the first century. This ought to be beyond debate. For one thing, because general conditions cannot be extrapolated from one particular case, the objectionable interpretation of Luke 4 would be utterly invalid even if those events (as portrayed by Luke) are fully accurate in their representational truth. Just because such a thing might have happened once is NO evidence that such a thing was typical. To the contrary, with storytelling, it is far more often the exceptional kinds of events which tend to be remembered and narrated. More importantly, the massive amount of research into first century Judaism reveals hundreds of positive reasons to form a different picture of Synagogue ethos at large. The milieu of first century Judaism may have been diverse, but the customs they all had in common were healthy and positive, albeit sometimes peculiar - as discussed in this online piece by A.-J. Levine. If nothing else, the recent emphasis on diversity should demonstrate that the behavior in Luke 4 - if depicted with accuracy - would have been severely atypical, to say the least.
Point Three: Luke's particular story was presented to his audience with some degree of plausibility. That is, the reaction of Jesus' hometown synagogue is portrayed as a natural event not-entirely-outside the realm of expected potential behavior. In the imagination of Luke's audience, the writer assumed, this extreme event would not be viewed as entirely absurd. However atypical - and we cannot underscore too strongly that it would have been severely atypical - the writer and audience must have shared some basis in reality for believing that the political disposition of at least some members of some rural synagogues in the first century might have been strongly opposed, - if not socially hostile, albeit not physically violent - toward the idea of including uncircumcised gentiles (non-converts) as full recipients of the blessings once promised to Abraham's seed.
To be clear, the scenario in Luke 4, while extreme, is more likely caricature than fantasy. Even as fiction, the story is being represented with some degree of realism as its contextual basis. Even as fiction, Luke would not expect his audience to be shocked that synagogue members would react negatively to the Christ-based imposition of gentile equality. Shocked to hear they tried to kill him? Surely. But shocked to think there was negativity toward that idea? Apparently not. Even as fiction, the story works because it builds upon basic plausibility. (Also, by the way, if the story is non-fiction, all these points would be equally true.)
Point Three, again: It was plausible for first century people to suppose that the some members of any given synagogue would be opposed to full inclusion of uncircumcised gentiles.
This qualified and measured assessment of plausibility in Luke 4 brings me to my point.
Arguments about the date of James' Epistle tend to hinge on his silence about gentiles. Supposedly, if James had heard about Paul's churches, he would have certainly mentioned those gentile believers. This, of course, depends on the assumption that James was writing to congregations of Jewish-christians across the Jewish diaspora. Admittedly, when working from that assumption, it is nigh impossible to imagine why James would fail to mention the acceptance of uncircumcised gentile believers, if the letter was written after the council of Acts 15.
An alternative hypothesis should be considered, however, based on a different assumption about audience. If we take James' opening lines of address at face value, then he is actually writing "To the twelve tribes dispersed". Why would James do that? My guess is public relations.
On the basic gist of Acts 21:17-26, James and the church in Jerusalem were concerned to present themselves as faithful keepers of Torah. Note especially, here, that even if that passage tells us nothing about James' opinion of Paul's ministry, it still purports something about James and the Jerusalem church. Whatever James did or did not think was appropriate for Paul's churches in far away towns, that's a separate discussion. My point on this passage is that James was concerned for the public perception of his own group and that public's perception of all those who were obviously associated with them. That included Paul on one occasion, according to Acts 21 (c.57 CE), and so it could have included any christians in Jerusalem between 33 and 66 CE.
If we suppose James sent his epistle, quite literally, to the twelve tribes dispersed, then the audience was Jewish but not christian, and this would explain lots of things very nicely. This would explain why James identifies himself as "a servant of the Lord Jesus Christ" but says nothing about the cross, the resurrection, or the holy spirit. It would also provide nuance to 2:1, which may thus be translated, "My brothers, do not with partiality regard the faith of our Lord Jesus Christ of glory." That is, the possessive sense of "exw" (have, hold) does not necessarily imply that James' addressees are themselves personal holders of faith in Christ. Rather, if we remember the addressees are in fact Jewish synagogue members, we should understand they are obviously not christians. Therefore, the essential meaning must be, "do not discriminate against the faith of our Lord". Note also the "hmwv" (our) applies to James and his fellow believers in Christ. There is no indication here that "hmwv" here is meant to include the letter's recipients. This reading also helps explain why 2:2 talks about people walking into their "synagogue".
If we read the entire epistle this way, a lot of reasonable connotations begin to present themselves. The notorious contradiction of Paul, for example, cannot be read as a direct refutation aimed at Paul's churches; rather, it is a corrective reassurance being offered to a general public who has probably heard negative reports about Paul's libertarian message. (Whether James would have offered the same words to Paul - in rebuke of Paul, deliberately in Paul's own terms - is a separate discussion.) More mundanely, the passages about wealth and poverty can be read as alluding to James' own economic dilemma, that the christian-Jews in Jerusalem had become poor due to various prejudices against them in the wake of Jesus' and Stephen's deaths. More generally, passages about wealth and poverty - like other passages in James that echo the words of Jesus in the Gospels - would seem more naturally addressed to a strictly Jewish audience, like those to whom Jesus had preached.
To the initial point raised above, this reading would certainly explain why James does not mention gentiles.
If James' purpose was to improve public relations, then it behoved him to demonstrate that the Jews who followed Jesus were indeed good Jews who had not forsaken Moses, but still cared about purity and observing the law. To this end, the letter acknowledges the author's group affiliation in 1:1 and makes a brief appeal for non-discrimination in 2:1, but otherwise keeps to a litany of exhortations which ostensibly represent an appropriately Jewish message, roughly along the lines of Jesus' (by now famous) preaching in Galilee and Judea. James' primary goal is to elevate the status of Jewish-christians in the eyes of Synagogue members everywhere.
If he tried to do that, while talking about gentiles, it would simply run the risk of spoiling everything.
Note well: I am not saying it *would* have spoiled James' plan. I am not saying James expected that it definitely would have hampered his goal. What I am saying is that James had to anticipate the likelihood of it being a risk. Thus, even assuming James did have a desire to mention positive things about gentile christians (and I say that despite my own hunch that he probably didn't), there would still be a much greater need to avoid saying such things. Especially if rumors were already flying around about Paul's message, after Galatians.
Finally, if someone wants to object that James should mention the controversy over circumcision when writing after the council, I would simply repeat all these points once again. There is no good reason to air out the gory details of internecine disputes during a public address aimed at improving opinions at large.
In summation, the primary argument for dating James before the council of Acts 15 is misguidedly dependent on a hermeneutically gerrymandered inference of a christian audience. However, if James' audience was Jewish synagogues, as I have argued, then the plausible risk of inflaming racial tensions - as illustrated by the underlying dynamic which atypically leads to violence in Luke 4:25-30 - explains why James does not mention gentile believers in his public relations project.
All of this, by the way, puts the letter more likely after Galatians than before it. But the date of Galatians would obviously be yet another discussion.