Are you sure that the phrase wasn't more or less invented by Jack Higgins for his novels?
Higgins himself gave this explanation in a letter:
"Judas gates constantly surface in my book and people have often commented. There is a painting, I think Victorian, which shows a very large double gate. Inset in this gate, is a small door or gate which you can open and step through without the inconvenience of opening the larger double gates. In the painting, Judas is seen stepping through on his way to betray Christ. So, in English factories, you find such a small door set in most big factory gates and they were traditionally known in Yorkshire, a very industrial area where I grew up, as Judas Gates..." (cited here https://english.stackexchange.com/qu...-of-judas-gate)
A "judas" (aka "judas hole" or "judas window") is a door in which an aperture has been cut for observing the person - usually a prisoner - inside; i.e. a spyhole or a peephole. This particular term has been in use since the early 19th Century.
The rest of the discussion you posted shows there is some doubt about how widespread this phrase was. Perhaps it's use was popular on the streets of Yorkshire where he grew up, but doesn't seem to have made it into print very often, outside of his books. You may have better luck, but I can't find any mentions at all in Google books between 1850-1900 (Judas hole, yes, Judas gate, no) nor can I find any in the press reports here.
Last edited by Joshua Rogan : 07-31-2018 at 01:24 AM.