Here is an example of how social context influences what we call behavior — whether we see it as mental illness. From the CBS new website (4-12-04):
"Saturday was al-Arbaeen, the 40th and final day of the mourning period for Imam Hussein, grandson of Islam's Prophet Muhammad, who was martyred in a 7th century battle. Ceremonies last until Sunday night. Shiite Muslims flailed themselves with chains and slashed their heads with swords, staining their clothing with blood as they mourned for Hussein, one of their most revered saints. "
I think most of us would not understand this behavior and would see it as "sick," "crazy," "whacko," "Not playing with a full deck," etc. At a professional level it is seen as a serious symptom of a psychological disorder. This illustrates that what we call "abnormal behavior" is for the most part simply behavior we don't understand. (see Self Mutilation).
But in the context of religious belief this self-inflicted torture is seen as normal and even commendable. These people are pleasing God; they are showing their devotion.
In the same way, the barbaric Christian ritual of eating Christ's flesh and drinking his blood (it's not just symbolic to orthodox Catholics!) is actually considered a sacrament — something that a religious person would take part in. How weird is that?
And how about this? The foreskin of Jesus when he was cirmcumcized for a long time was considers a sacred relic. To me it seems an egregious perversion to worship the foreskin of a penis! (see Holy Prepuce, Batman!)
You can see that before you can call a behavior a psychological disorder, you must first determine the context in which it occurs.