Cretinist is a portmanteau of cretin and creationist. It's entirely derogatory, obviously. As such, some rationalists would argue it shouldn't be used on account of its ad hominem properties - it's basically on par with "evilution" as far as bad, derogatory puns are concerned. Others argue that it's just a snarky way of calling a spade a spade.
Surprising etymology
In current English usage a cretin is "a stupid, vulgar, or insensitive person".[1] Historically it is from the French word crétin, a wretch, innocent victim. Which is in turn from the Latin christianus or Christian. So when you are calling someone a cretinist you are not in fact insulting them, you are calling them a Christian creationist.
There is also a real disease called cretinism, which potentially adds another layer of poor taste. [2]
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Footnotes
Adapted from an out of date article at RaonalWiki