How Does a Priest Distinguish Between a Person that is Demonically Possessed and a Person that is Mentally Ill?
A person cannot simply go to a priest, declare they are possessed and have an exorcism performed on them. The process of exorcism is not how it works in the movies. There are a lot of steps a priest has to follow before declaring that a person is demonically possessed. They have a team of psychologists that look at the person and only when they write out all natural explanations will the priest continue down the path to see if demonic possession is a possibility. A priest will have to look for supernatural evidence and put the person under certain tests to see how they respond. Only after these steps, will a priest declare if a person is demonically possessed or not.