I think that cretin is an ex-medical term referring to the physically and developmentally stunted.
[EDIT]
I should have read the article I posted, as the ‘Christian’ etymology you described is given there, although I still find the justification rather forced. the explanation linking it to ‘cretine’ - alluvial deposits, makes more sense - particularly as iodine levels in the soil are linked to the prevalence of the condition.
