What does Mark 9:21 mean?

Epilepsy is a neurological disorder that causes recurring seizures. Convulsions and foaming at the mouth are common, and if the condition appears early enough in the person's childhood development, the language center of the brain can be affected, making it difficult to learn a language. That doesn't mean that epilepsy, in general, is caused by demonic possession and, in fact, Matthew 4:24 differentiates the two conditions. When Jesus heals, the Gospels are careful to distinguish between a physical condition and demonic possession. In this case, the boy is clearly possessed by a demon that apparently causes seizure-like manifestations. In Matthew 17:15, the father diagnoses the boy as physically disabled, or epileptic. Jesus treats the boy as demon-possessed and nothing else.

This is not the first time the Bible talks about the duration of a condition. Mark states the woman with an issue of blood suffered for twelve years (Mark 5:25). Luke, the physician, notes that a woman had a disabling spirit for eighteen years (Luke 13:11). John speaks of one man who had been an invalid for thirty-eight years (John 5:5) and another man blind from birth (John 9:1).

God does not promise physical healing today, but if He chooses to heal, He is not constrained by the duration of the condition. He is willing to heal our hearts no matter how long ago or how enduring the abuse, trauma, or sin has been.