Mark 9 : 17, 20, 26, 28
17 Then one of the crowd answered and said, "Teacher, I brought You my son, who has a mute spirit.
20 Then they brought him to Him. And when he saw Him, immediately the spirit convulsed him, and he fell on the ground and wallowed, foaming at the mouth.
26 Then the spirit cried out, convulsed him greatly [...].
28 And when He had come into the house, His disciples asked Him privately, "Why could we not cast it out?"
The disciples could not cast out a "mute spirit". They wanted to, but could not, because it was not the intention.
They did receive the power - and occasionally also the assignment - to drive out evil spirits.
The fact that in this case they could not, is related to the deeper meaning of this story.
The miracle of casting out is a sign and therefore, above all, has a prophetic and typological meaning; initially for Israel,
but also for humanity in general and for individual human beings.
This Bible study addresses this and focuses on the position of the believer in this dispensation. The Lord is not on earth, but is beyond the scope of the world in his glorious position on the "mountain".