Going back to an earlier comment –
Actually, the Roman authorities would be concerned with arresting Jesus for claiming kingship, interested with a claim of prophecy (in case Jesus predicted the Emperor’s death, which was illegal), but would not care at all about Jesus claiming to be a god.
It was not illegal to claim godhood in the Roman Empire, as long as you didn’t attack the State. Lots of people claimed to be gods or goddesses, or to be possessed by them. Happened all the time in the pagan world. The Emperor was a god in a lot of Asian cities of the Empire, with temples and everything! So were a lot of members of the imperial family. In Rome itself, they held back during early times until somebody died, but soon there was deification in Rome of the living, too.
Now, I suppose they’d have a case for disturbance of the peace, since the Jews had strong views about gods. But that’s nothing. Claiming kingship of a land inside the Empire? That was treason.
So yeah, when the Montanists claimed that one of them was the incarnated Holy Spirit, the Roman government didn’t care one little bit. Much safer to claim godhood than claim to be a Christian, in the wrong place at the wrong time.
And going back to the OP’s question –
The OP ought to look up sources discussing the heresy of Adoptionism. If you want to argue historically, you still need to look up the theological sources. (You will also find better footnotes and sources that way.)
But the first place to look is whether Ehrman’s sources for the quote have any validity. If Ehrman doesn’t have any decent sources listed (which wouldn’t surprise me), you can then go look for your own. I don’t have Ehrman’s book, and you didn’t even say which book it is or where in the book. So it’s difficult to do the spadework for you.
Ehrman is very slippery. I still can’t forgive the dishonesty of him claiming that St. Epiphanius’ entire book on heresies was lies, especially the part where Epiphanius talked about a heretical group trying to seduce him into their weird sex practices when he was a kid – and then, later, silently quoting Epiphanius about the group, because Epiphanius is the only source that knew much about them. (For the record, St. Epiphanius’ account of the doings of this sex cult rings true to anybody who is a true crime buff, or has met iffy occultists of that sort. But yeah, don’t believe the victim.) It’s not “fraud,” but it’s not good scholarship or honest argument.