A little while ago, I posted a criticism of an argument from Bart Ehrman's Misquoting Jesus: The Story Behind Who Changed the Bible and Why. I said I had one other criticism to post and am now finally getting around to writing up my thoughts on it.
John 1:18 calls Jesus either "the unique Son" or "the unique God". Ehrman (on pp.161-162) argues that the former reading is more likely. He admits that the second reading is found in the Alexandrian manuscripts, which are generally regarded as closer to the original biblical texts. That would normally be a decisive enough argument for me, in lieu of some other consideration that would nevertheless make it less likely to be the original reading. Ehrman thinks he has such a consideration.
One interesting piece of evidence is that you don't find "the unique God" very much in non-Alexandrian manuscripts, which is some indication that it either came into the manuscript tradition after the Alexandrian branch diverged (Ehrman's view) or that the change to "the unique Son" was so early that it managed to get into most of the other manuscripts that survive. I certainly wouldn't rule that out (and its occasional presence does undermine this argument a little bit), but this is one piece of evidence against "the unique God" being original.
Even so, Ehrman raises some other arguments against "the unique God" that I can't agree with. He says John uses "the unique Son" elsewhere but never "the unique God". I suppose that counts for something, but there's nothing to rule out John using an expression in the very different prologue that never occurs elsewhere in the gospel.
The argument that really baffles me, though, is his claim that "the unique God" makes no sense when applied to Jesus. Only someone who isn't thinking in terms of classical Trinitarian theology could say such a thing. He says the term for "unique" in Greek means "one of a kind". So far so good, until he concludes that such an expression must therefore refer to the Father and not to Jesus. "But if the term refers to the Father, how can it be used of the Son?"
Obviously, the people he believes to have changed the text thought it meant something that they thought they understood, or they wouldn't have changed it for the ideological reasons he thinks they changed it for, so there's immediately something suspicious about his claim that both of the following are true:
1. It was changed for ideological reasons because the changed text better supports proto-orthodoxy.
2. What it was changed to makes no sense when applied to Jesus and violates proto-orthodoxy by applying something true only of the Father to Jesus.
Consider what the view he calls proto-orthdoxy holds. The classic Trinitarian view is that Jesus is God. There's only one God, and both the person called the Father and the person called the Son are that God. So any characteristic of that God, say his uniqueness, is true of both the person called the Father and the person called the Son. In light of that, doesn't Ehrman's argument sound very strange? He has to assume from the outset that classic Trinitarianism, the very proto-orthodoxy that he thinks this change was introduced to support, is not a viable view, or he couldn't claim that it makes no sense. It makes perfect sense according to that view, and it's exactly the sort of thing you'd expect someone holding the view to accept as true.
Finally, if "the unique God" is theologically puzzling, as it might be for a copyist who doesn't fully grasp classical Trinitarian theology, then we have a perfectly good explanation of how a copyist might have gone from "the unique God" to "the unique Son" if the latter is indeed more easily understood, as Ehrman says it is.
Maybe some of the other reasons might be good reason to withhold judgment on which reading is earlier or maybe even to favor "the unique Son". I'm not doing a comprehensive look into which reading is likely to be better. I just thought this particular argument is especially problematic, and yet it seems the one Ehrman takes to be most decisive.