‘And behold, a man from the crowd cried, saying, “Teacher, I beseech you to look on my son, for he is my only child, and behold, a spirit takes him, and he suddenly cries out, and it tears him so that he foams, and it hardly departs from him, bruising him sorely. And I begged your disciples to cast it out, and they could not.”
A man speaks to Him from the crowd. He describes how he had brought his only son to the disciples seeking help. His son was possessed by a spirit which spasmodically made the child cry out and then tore him with the result that foam came from his mouth. And this happened more often than not and caused him great distress. But despite their efforts the spirit resisted the disciples and they could not cast it out. That it was not just epilepsy comes out in what follows. A mere disease would not have resulted in failure for the disciples, nor would Jesus have spoken it as requiring special power.
‘He is my only child.' The man's only child was continuing to suffer because of the failure of the disciples. Luke 9:41 reveals how God's only Son was also suffering at their hands.