Not being a Christian, I was (and still am) relatively unaware of Bible and its teachings, except for a few well-known stories, heard or read in vernacular at a young age. Once I started with English literature, I used to be stumped by many an idioms, most of these being biblical allusions! In the days before Google, it was not easy to find many references. This problem became pronounced once I started reading P. G. Wodehouse. Just like in your example of “transfiguration”, while I was able to ‘get’ the meaning, the references to something as abstruse as “scales falling from eyes” was jarring. Did he mean ‘scales’ as in ‘weighing scale’ or scales as in ‘a fish has scales’?!
English language is replete with such references, even going back to Greek Mythology and Roman literature, but mainly Biblical references, as I have come to learn. To the native speaker of English language, it may not be so evident. But to me, someone who has learnt English as second or third language, with added unfamiliarity with Bible, its stories and parables, it does present a challenge. While PGW may use these references to enhance the humor, many other authors like to convey a deeper, hidden meaning. Missing the gist of it, definitely limits the appreciation. Hence, I wholeheartedly agree with your premise.