I reject the premise - one could better say that Latin was read and understood by the elite class, not the common man, and by the time frame we are talking about, Latin was already highly bifurcated into Spanish and Roman (Italian), with heavy dialects (Occish as an example) separating the two where they would tend to meld. So even in the Latin speaking areas, pure Latin would not readily be understood by the common man... And that still does not take into account the germanic and celtic regions in the least.
Equally against your statement is the apparent zeal with which the Word was received any time it did in fact make it into the common tongue in any region.
But what has that to do with your previous statements anyway?
I am saying that Latin served as a common bond to pull together Christians of the West. Just as Greek pulled together the Early Christians. This is the Latin of the educated. I doubt that the average Roman of the 4th century would have understood Cicero much better than the average Roman of the 14th Century. But it made it possible for an educated Roman to correspond with an educated Londoner. Furthermore, even though a Frankfurter and a Zuricher speaking Latin might have had very different pronunciations, it was probably no more difficult than the two trying to converse in German when there was no such thing as standard German.