>From _You English Words, A Book About Them_, by John Moore:  speaking of 
>"my old schoolmaster", Moore writes:
>
>       "Cretin" was another of his favourite terms of abuse, especially 
>     when we were doing Latin Unseen; so it was only fair that in calmer 
>     moments he should explain to us how it came about that the French 
>     word for a Christian got the meaning of "congental idiot."


Looks like my trusty "American Heritage Dictionary of the English
Language" backs you pretty well on this.

cretin n. 1. One afflicted with cretinism.  2. An idiot.
          [French "cretin", idiot, from Swiss French "crestin",
          Christian, hence human being, hence deformed idiot (who
          is nonetheless human), from Latin (Christianus, CHRISTIAN.]

Somehow, I had always suspected this.