How should a Christian respond to mental illness?

Question from a reader:

How should a Christian respond to mental illness? How do you determine if there is demon possession or if it is genuine mental illness, in which case, the person is still acting in sinful ways based upon biological problems. Of course some mental illnesses are said to be chemical imbalances but I’m more curious about our response to severe mental illness. Also, closely related to mental illness is brain damage victims and a change in behavior as a result. How is the Christian community supposed to respond to sinful behavior that is a result of brain damage?