Note 1 at Mt 17:19: This was a valid question from Jesus' disciples caused by prior success in casting out devils (see note 1 at Lu 9:41). If they had not believed that they could cast out devils and if they had not already seen that faith produces deliverances, they wouldn't have been surprised and confused at this result. They did have the faith necessary to cast this devil out, and they knew that (see note 2 at Mt 17:20).