There is a point in Galatians that I think is a strong clue of authenticity of that epistle.
I mean the mythicist interpretation of Gal 1:19 : Paul is going to secure the Galatians that he saw only a simple brother, James, and only an apostle (probably already a Pillar), Peter, shortly too, so he, Paul, is a true apostle from the start.
Who has read Detering on Galatians realizes easily what is the interpolation argument:
the proto-catholic interpolator inserted the first visit to Jerusalem (by having in mind Acts) in order to make Paul a factual servant of Peter in first place, and a banal observer of the simple ''fact'' that James is brother of Jesus (a descriptive 'fact' as well as is descriptive ''fact'' Gal 4:4: ''born by woman, born under the law'') in second place. So, contra Marcion, Saint Paul is both dependent on Saint Peter and a strong witness of the biological family of Jesus by simply ''seeing'' James. The interpolator used the verbe ''to see'' in Gal 1:19 to emphasize that is a simple 'fact' that Jesus had a biological family, just as is a simple 'fact' that Jesus was ''born by woman'').
But I have a problem with this argument.
In Gal 2 James is put implicitly in a negative light: the ''some people came from James'' are false apostles for both the Paul of Marcion and the Saint Paul of protocatholics. The proto-catholics and marcionites were both enemies of excessive-Judaizers as the ebionites. It's expected that Marcion could put the ''James party'' in a negative light. But is it expected in equal degree that proto-catholics could do the same? Could a proto-catholic tolerate the fact that Saint Paul despised the 'so-called Pillars' (among them: James brother of Jesus) because of their excessive Judaism? Could a Catholic sell so easily James as icon of the ebionites?
The Christian apologist Bernard thinks that the brothers of Lord weren't Christians. I don't accept his arguments a priori, but I think that he, as pious Christian, gives strong evidence that he doesn't like the latent hostility between Paul and James in Gal 2. If he doesn't like that fact, even more so the proto-catholics didn't like it, too. In other terms, I'm saying that Detering fails partially to explain why the proto-catholic interpolator didn't change the negative show of James in Gal 2. After all, he was the brother of Jesus!
Maybe a possible objection by Detering may sound so: the mother and brothers of Jesus in the Gospels are negatively represented, so the Catholic interpolator could be justified 'still' in the epistle to introduce a semi-negative James brother of Jesus to use against Marcion. But this 'solution' does not seem satisfactory, since in incipit of Acts ''his mother and brothers'' are Christian converts, so it is assumed that no longer they consider Jesus a madman (therefore no reason more for a late Catholic interpolator to denigrate James basing on Acts). And Acts doesn't mention no James brother of Jesus that is Pillar of Jerusalem, too.
The historicist argument (assuming an authentic Galatians 1 & 2) is well described by Zeba Crook when he says that:
Crook claimed Paul “wished” James wasn’t the brother of Jesus (because that made James a greater authority than Paul).
On the same frequency, Loren Rosson finds a clue/reason to doubt about Carrier's mythicism when he writes:
Carrier is being a bit obtuse here. No one, least of all Crook and Goodacre, is leaning on Christian faith doctrine; this is a scholarly construct based on objective assessments of Paul’s relationship to James and the other pillars. Even if you know nothing of Acts 15, it’s not hard to see the power struggles implied in Gal 1-2. (As an aside, I even suggest that James used his authority treacherously.) This is a feeble swipe on Carrier’s part and one of his least persuasive arguments.
https://rossonl.wordpress.com/2014/06/2 ... -theories/
A ''James brother of Jesus who made virtually him a greater authority than Paul'' is the same reason that may move a Catholic interpolator to insert Gal 1:19 in a former marcionite epistle to catholicize it. Therefore the historicists seem to think as proto-catholics.