A cretin, in the original sense, is a
Christian.
Webster 1913 defines cretinism as
A condition of endemic or inherited idiocy, accompanied by physical degeneracy and deformity (usually with goiter), frequent in certain mountain valleys, esp. of the Alps.
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Nowadays it's more generally used as a derisive term for a person with inferior intelligence, and not necessarily this specific form of physical and mental retardation. Well enough, you say, but where did it come from?
The Alpine peasants with whom the term originated didn't get enough iodine in their diets. Strange and unhealthy things often happen to children whose mothers suffer from iodine deficiency. That's why the salt you buy at the store is iodized - so the poor who can't afford a proper diet have a lower risk of giving birth to retarded kids. Unfortunately, mountain rustics in medieval times didn't have access to this now-trivial triumph of medicine and technology, and so their offspring contained a disproportionately large number of idiot dwarves.
Christian doctrine holds that the weak and inferior are to be pitied. The priests in these mountain villages reminded their parishioners of this, explaining that their defective progeny were still human and had souls, which distinguished them from mere animals. To the isolated and illiterate villagers, the words "human" and "Christian" were synonomous, and so the village idiots became known as Christians, or in the local dialect, cretins.
Though the etymology of cretin is largely forgotten today, there is a Catholic high school in St. Paul, Minnesota known as Cretin-Derham Hall. Having become coeducational after a merger of boys' and girls' schools, it was originally the all-male Cretin High School, founded by the Christian Brothers in 1871.