There is a somewhat strange episode in the Gospel of Mark (okay, so all of Mark is strange) where a father brings his son who has a spirit that makes him unable to speak and causes him to fall on the ground foaming and grinding his teeth (this sounds like epilepsy, but it is misnamed as such since we are dealing with an entirely different disease etiology in the text). The problem is that Jesus’s disciples are unable to cast out the spirit and so Jesus does it himself. Leaving aside the issue of demonic possession and medical diagnoses, the strange part of the story is the saying of Jesus that concludes the passage.
When the disciples ask in 9:28, “Why could we not cast it out?”, Jesus responds by saying “This kind (γένος) can come out only through prayer.” The first problem is that Jesus doesn’t pray to cause the spirit to come out. Why not? In the story, the reason that the spirit comes out is because the father believes. Then Jesus rebukes the spirit and it eventually comes out. It is worth noting here that both Matthew and Luke drop this ending of the story. In Matthew, the disciples still ask the question about why they weren’t able to cast it out and Jesus says that they lack faith. In Luke, the powerlessness of the disciples is dropped entirely. But what is the point of the story in Mark? Is Jesus more powerful than the disciples so he doesn’t need to pray? Or, can we explain the text by saying that the story was conflated with the saying?
The second strange part of the story is the text critical problem that some manuscripts add “and fasting” to the end of the verse. Modern translations and critical editions treat this as a later addition. Of course, Jesus doesn’t fast either, so the addition of this instruction doesn’t solve the problem with the story that Matthew and Luke saw. If anything, it makes the problem worse. So why add this to the verse? Could it be that the later disciples were trying to cast spirits out of people having severe medical problems and it wasn’t working with just prayer, so “fasting” was added as a requirement?