An excellent piece by Thomas A. Robinson at the University of Arizona's Biblical Interpretation website, an underutilized source for biblical criticism.
Thesis is stated right at the top:
The widely accepted “god-fearer” thesis rests on weak foundations: the ancient terms are not clear technical labels, the evidence is sparse and often overstretched, and the model relies too heavily on assumptions about synagogue-associated Gentiles supposedly primed for Christian conversion. Early Christianity did not require a large class of literate, well-connected god-fearers to explain its growth, and this thesis has significantly shaped, and likely distorted, modern reconstructions of the movement’s origins, membership, and leadership.
Followed by five main points, all apparently taken from a book he wrote on the subject last year.
The first point delves deeper into the "labeling" issue, looking at the two most common terms, and how and where in Greek they were used in the Septuagint. He concludes that by noting, while "god-fearer" is technically correct, whether as a dynamic translation we should even use that.
Second point? He says there's little evidence of the phenomenon from Jewish, Christian or pagan literature.
Third? He references the "Christianizing" of the Septuagint — including an overreading of how much, vs how little, Paul directly uses it, as a veil of sorts (my idea, riffing on Paul talking about the veil of Moses and analogizing from it) over clearer understanding of the LXX on this issue. He notes that the vast majority of the population was illiterate and so had no familiarity with it anyway.
Fourth point plays off the third and gets directly to Jewish vs Christian interpretations of the phenomenon, and larger Christian vs Jewish differences. He says, or indicated, there's a lot of assumptions at work here that have little standing. One is that Christianity would have been perceived as the only option for any actual god-fearers when a Christian preacher showed up. Another is whether, or not, circumcision was a real hurdle. Third is that conversion to a tiny cult would likely have been offset by a loss of Jewish friendships.
Fifth is the question of how long any significant body of god-fearers were around as a potential tool of converts.
Finally, his conclusion starts with this:
Almost every detail of popular and scholarly reconstructions of early Christianity is affected by the god-fearer thesis, from an emphasis on the economic, social, and intellectual resources of its membership and leadership, to its urban (and urbane) character, and its Jewish-tutored background. Such focus on god-fearers as a vital component in the early Christian movement tends to overlook how flimsy the evidence for god-fearers is and how unnecessary the god-fearers are to the developing Christian movement.
Give the whole thing a read. It's worth it.
Assuming he's right, this means that large swathes of Christianity would have been of a fully Hellenized, or Hellenized/Syriacized (sic) nature by no later than, say, the time of Marcion if not a full generation earlier, without "godfearer" intermediaries. OR, it means, given that we have moved well beyond Josephus on varieties of Judaism, that even more of Christian metaphysic than we have thought has Jewish roots. This, in turn, has implications for Christian development of Christology and related matters.