Usually a fundamentalist, holier-than-thou, Bible-thumping, Scripture-quoting, Hallelujah-ing Christian, who often claims to be a "creation scientist" despite obviously bad science and flawed logic. Outspoken and eloquent, they can be very persuasive to the uninformed and pose a grave danger to our public school systems. Someone who naively and blindly believes in what one ancient book (Genesis) says and ignores the huge mountain of evidence and the power of human logic attesting to the rise of man and other species by naturalistic (sans divine intervention) mechanisms. Some also still believe that the earth is flat.
A notorious cretinist is "Dr." Kent Hovind who bought his degree from a diploma mill.
Suggested reading: The Selfish Gene, by renowned Oxford biologist Dr. Richard Dawkins. This is one book that, if you can uderstand the complexities involved can change your whole worldview forever as it did mine.
Beware, a cretinist may be coming to a church or PTA meeting near you!