I think that the “Take our Word for it” entry (Feb 1999) is good.
While the original meaning of cretin was, literally, “Christian”, the word “Christian” was not being used as we would use it today. In our pluralistic, multi-cultural society, we recognize Christianity to be just one of many competing belief systems. Thus, to say that someone is a Christian is to state that s/he is not a Buddhist, a Marxist, a Hindu or a Jew. This seems rather obvious to us, doesn’t it? Yet this wasn’t quite how the word was understood by the medieval inhabitants of remote Alpine valleys. From their limited and parochial perspective it seemed that everyone in the world was Christian. Thus, the word became synonymous with “human being”.
But then they end with this:
We must admit that we were quite surprised to find that the word Christian itself was not used in English until 1526. How did English-speaking Christians refer to themselves before that date? Did they not need such a word before they came into contact with non-Christians?
This makes me suspicious as the word was used in the book of Acts to mean the earliest followers of this faith. That, of course, was in Greek (Χριστιανός) and not in Latin which seems to be the etymon for the modern Cretin. The Vulgate used “Christianum” at Acts 26:28. So, I don’t know.
The understanding I have of the derivation of chrétien > Cretin was to suggest that these folks were innocents incapable of sin is moving, but still suspect in my view.
I’ve never heard the derivation of “people thus afflicted might look and behave like beasts but were nevertheless baptised Christians. “ before.