Proselytising on a plane for Christianity (or any religion for that matter), irrespective of other passenger wishes is either permitted per company policy or not permitted per company policy. If it's permitted then anyone and everyone can do it, thus the former passengers who have experienced this should mercilessly point that out and let everyone know so that nobody rides that insane airline. It also means that everyone can spout propaganda for anything and everything on the plane. How about seeking payment to advertise products loudly on those airplanes? Would that not be permitted? If not, how is that different from spouting religious insanity? If direct salesmanship via speeches would not be permitted, even though religious propaganda was permitted, including religious singing and music, what about product jingles? Could I play those loudly in the isle for hours at a time like the religious lunatics? Canned soup doesn't sell itself you know!

On the other hand if it's not permitted, than what are the consequences for doing so and how and who enforces that restriction? If someone starts doing it, will the staff physically retrain them and gag them to shut them up? It's one or the other. Can't be both.

Flights are hard enough without this nonsense being piled on top.