And he said unto them, This kind can come out by nothing, save by prayer.

And he said to them, 'This kind is able to come forth with nothing except with prayer and fasting.'

"An evil spirit of this kind," He answered, "can only be driven out by prayer."

And he said to them, Nothing will make this sort come out but prayer.

And he said to them, "This kind is able to be expelled by nothing other than prayer and fasting."

"A spirit of this kind," he said, "can be driven out only by prayer."

Prayer and fasting - See on Matthew 17:21 (note).
This demon may be considered as an emblem of deeply rooted vices, and inveterate habits, over which the conquest is not generally obtained, but through extraordinary humiliations.
This case is related by both Matthew and Luke, but it is greatly amplified in Mark's account, and many new circumstances related. Another proof that Mark did not abridge Matthew.

And he said unto them,.... Matthew, besides the following reason, assigns another, as given by our Lord, why they could not cast out the foul spirit, which was their unbelief; of which they were guilty in some sort, as well as the Jews, and the father of the child; but Mark omits it, and only relates this as the reason;
this kind can come forth by nothing, but by prayer and fasting; which they had not observed; See Gill on Matthew 17:21.

And he said unto them, This kind can come forth by nothing but by prayer and fasting--that is, as nearly all good interpreters are agreed, "this kind of evil spirits cannot be expelled," or "so desperate a case of demoniacal possession cannot be cured, but by prayer and fasting." But since the Lord Himself says that His disciples could not fast while He was with them, perhaps this was designed, as ALFORD hints, for their after-guidance--unless we take it as but a definite way of expressing the general truth, that great and difficult duties require special preparation and self-denial. But the answer to their question, as given in Matthew 17:20-21 is fuller: "And Jesus said unto them, Because of your unbelief. For verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place, and it shall remove; and nothing shall be impossible unto you" (Matthew 17:20). See on Mark 11:23. "Howbeit this kind goeth not out but by prayer and fasting" (Matthew 17:21), that is, though nothing is impossible to faith, yet such a height of faith as is requisite for such triumphs is not to be reached either in a moment or without effort--either with God in prayer or with ourselves in self-denying exercises. Luke (Luke 9:43) adds, "And they were all amazed at the mighty power of God"--"at the majesty" or "mightiness of God," in this last miracle, in the Transfiguration, &c.; or, at the divine grandeur of Christ rising upon them daily.
Second Explicit Announcement of His Approaching Death and Resurrection (Mark 9:30-32).