In case anyone else was also pondering the difference between "cretin" and "idiot" (I can't be the only one) I thought I would share the following information regarding various types of mental retardation.
Idiot indicated the greatest degree of intellectual disability, where the mental age is two years or less, and the person cannot guard himself or herself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
Imbecile indicated an intellectual disability less extreme than idiocy and not necessarily inherited. It is now usually subdivided into two categories, known as severe mental retardation and moderate mental retardation.
Moron was defined by the American Association for the Study of the Feeble-minded in 1910, following work by Henry H. Goddard, as the term for an adult with a mental age between eight and twelve; mild mental retardation is now the term for this condition. Alternative definitions of these terms based on IQ were also used. This group was known in UK law from 1911 to 1959/60 as "feeble-minded".
I advise you to only use these terms when speaking to an individual who has accepted such a diagnosis from a medical professional. Otherwise, refrain from using this terminology around and with said "mentally challenged" due to the great likelihood that they will become severely defensive and lash out at you (have adult tantrums).
Now, as for getting said folk to stop posting embarrassingly stupid shit on a thread they can't even comprehend (because they are not reading it with an open mind)... That is a problem I doubt will be solved anytime soon.