John Coffey is from AzarathHis powers are empathic based and in order to heal someone he has to take their pain and put it inside him. Not only that but he empathcially feels the pain of everyone he is near, from minor disease such as Paul's infection, to what Wild Bill did.
Wild Bill has empathic powers of his own.Rather than healing people like John does. He revels in every physical emotional pain. Not only that but even after being drugged by Paul he was able to get catch John out of his Cell. Almost like he sensed him passing by.
John is somehow related to Abigail.John was one of God's true miracles, so it would make sense that someone closely related to him would be a prophet for God.