This fantastic guys, thank you ever so much.
The hermit is actually Christian, of course, not Jewish. I got that wrong! He's exiled on a Greek island, and the letters are to Christian churches in Asia. Aramaic or Greek sounds about right to me, I've been searching for information about this, but your help is brill. The stuff about stilus and parchment, and pointing out the stuff about the Roman Empire - really helpful. Thank you.
Any clue who the hermit is, anyone?
JB