J is for Justice
Justice: the quality of being just; moral rightness.
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There was one reason that John exorcised spirits. It was for the justice of the ghosts. The right that they had to decide their fate to go to Heaven or Hell, or to meet the Lord or the Devil. It was their own choice, and he was ready to grant them that decision.
But he had to be careful not to show any of his true motives to the church, who believed in removing all the 'evil spirits'.
He liked giving the ghosts the opportunity to redeem themselves, or re-find themselves. And he thanked Naru for allowing him that chance.