Wrong etymologies for Germanic (and Proto-Germanic) words

That may be true, but I doubt they would have used the specific Christian words until they became more integrated into their society. The Angles and Saxons living on the North Sea coast would not have had any significant contact with the Romans (their southern neighbours would have). Of course, they would have heard about it, but Christianity was probably just another religion practiced by walhôz and so they didn't really feel a need to talk about those things on a regular basis (we don't see any words referring to the pre-Christian traditions of Rome, either!). And when they did, they probably used descriptive or analogous terms, just like they did for many terms when Christianity did gain a hold, like "God"! Languages don't borrow terms until a foreign word is felt to add expressive power to the language, and the Angles and Saxons presumably didn't have much need for specific words for Christian concepts as those concepts were not important enough to them to distinguish.