Although we dont know for sure the devil exists, we do know for sure that jesus of nazareth existed and that he done many magical things, curing the sick, giving sight to the blind and even raising people from the dead we know that he was crucified and after 3 days the tomb was found empty this is wrote in both the jewish and roman history not just the bible.
All it comes down to when you read about jesus of nazareth in the gospels, when you know for sure he existed, you have to come to one of the following conculsions.
1. He was a mad man and someone stole his body under the noses of roman guards (who were unlikly to be asleep on duty - consiqunece is death)
or
2. That he was and is the son of god who he claimed to be and rose from the dead.
Well, I believe there is sufficiant evidence to suggest that he was the son of god not leastly the signs that he done and are documented in all three texts regarding him, jewish history, roman history and the 4 gospels. But also, the many healings that still occur today at shrines around the globe, fatima, lourds being 2 of the most well known.
If we can trust history, we can know where to look for more indepth info on jesus and when we do, we know that jesus healed people who were possesed by demons.
"then jesus rebuked the demon, and it came out of him, and from that moment the boy was healed."
"he healed many who were sick with various diseases, and drove out many demons."
"those tormented by unclean spirits were made well"
"with authority and power he commands the unclean spirits, and they come out!"
it is also reported in the records of the jewish temple
"the pharisees however, said, "he drives out demons by the ruler of the demons!"
so I think schizophrenia and many other mental disorders could easily be the attacks of unclean spirits although certain mind numbing drugs may help with the paranoia and the decrease the awareness of the voices, they dont actually cure they only alievate the problem.
I believe by turning to jesus christ and recieving the sacraments instituted by christ, through the catholic church that he created 2000 years ago is the one way of begin healed.
Perhaps a trip to loureds or fatima may also cure a believer in the one ture faith.