Luke 9:39

And behold, a spirit taketh him, and he suddenly crieth out; and it teareth him that he foameth, and it hardly departeth from him, bruising him sorely. And I besought thy disciples to cast it out; and they could not.

The page headings in the English Revised Version (1885) title this wonder, "The Epileptic Boy," due to the resemblance this condition had to that disease. Mark used a word which means "lunatic," but the same thing is done there. The apostles were describing the symptoms, not the cause of the malady, the cause of it being clearly revealed as demon possession. Jesus not only rebuked the unclean spirit, which could not be understood as rebuking a disease; but the other synoptics recorded Jesus' revelation that the particular demon in that case was unusually malignant. "This kind can come out by nothing, save by prayer" (Mark 9:29). Thus, whatever the symptoms, this was a case of demon possession.

They could not ... The failure of the apostles, in this case, was due to some failure within themselves; for it is written that Jesus had given them authority "over all demons" (Luke 9:1); and what was requested of the nine apostles by the distraught father was clearly within their commission. It appears, however, that they had neglected prayer; and there could also have been on their part a kind of self-reliance apart from the power of God, feeling, perhaps, that "in themselves" personally resided the power to do such things. As a consequence, they were embarrassed by failure. How often have men of all generations failed through not yielding their will to that of the Lord and by not seeking his continual blessing through constant prayers and supplications.