It is actually the father, not the disciples, that Jesus accuses of lacking faith here.
The account is in all three synoptic Gospels: Matthew 17:14-18, Mark 9:17-27, Luke 9:37-42. In Mark, Jesus is quoted as saying, O faithless generation (ὦ γενεὰ ἄπιστος); in Matthew and Luke, he says O faithless and perverse generation (ὦ γενεὰ ἄπιστος καὶ διεστραμμένη).
The word the NIV translates here as "perverse" is a passive participle of the verb διαστρέφω (diastrephō), which is defined in various lexicons as pervert, distort, divert, turn away, mislead, or lead astray. "Perverted" (not in the sexual sense) rather than "perverse" is probably a more literal translation. The root is στρεφω (strephō), which means to turn or turn around oneself. The prefix δια- (dia-) signifies thoroughness or completeness, so we might interpret diastrephō to mean something like throughly turned or thoroughly turned around.
Many other versions also translate diastrephō as "perverse", but it is also translated as "twisted" (ESV), "corrupt" (NLY), and "rebellious" (HCSB). Tyndale chose the word "crooked" (croked). "Perverse" is consistent with the Latin version of the text, which uses the word perversa here.
It is relevant here that Jesus is actually upbraiding the father, not the disciples or the crowd. The father is essentially committing blasphemy, finding fault with the grace by which Christ imparted power to the disciples to cast out demons (Matthew 10:8, Mark 3:15, Luke 10:19). This can also be seen in the rude and discourteous way that he approached Jesus, immediately blaming the disciples. Theophylact (1055-1107) noted that he even seems to blame nature (the moon) for his son's malady1: the Greek text of Matthew literally reads He is lunatic (σεληνιάζομαι - "moonstruck"). Commenting on the father, Cyril of Alexandria (378-444), wrote:
He did not simply ask the healing of the child, and in so doing crown the healer with praises, but, on the contrary, spake contemptuously of the disciples, and found fault with the grace given them. For I brought him, he says, to Thy disciples, and they could not cast it out. And yet it was owing to thy own want of faith that the grace availed not. Dost thou not perceive that thou wast thyself the cause that the child was not delivered from his severe illness?2
It was the father and not the disciples who lacked faith. We might recall here:
Matthew 21:22 (KJV)
And all things, whatsoever ye shall ask in prayer, believing [πιστεύοντες; also lit. having faith], ye shall receive.
(See also Mark 11:24)
Cyril also commented extensively on the nature of the man's "perversion":
It was the duty therefore of the father of the lad rather to lay the blame upon his own unbelief, than upon the holy apostles. For this reason Christ justly called out, O faithless and perverse generation: how long shall I be with you, and suffer you? He justly therefore calls both the man himself, and those like him in mind a faithless generation. For it is a wretched malady, and whosoever is seized by it is, as He shews, perverse, and utterly without knowledge to walk uprightly. And therefore the sacred Scriptures say of such persons, that their ways are crooked, and their paths perverse.3 From this malady the divine David fled: and in order that he may also benefit us, he reveals the set purpose of his mind thereupon, saying, A crooked heart hath not cleaved unto me:4 that is, one that cannot walk in an upright course. To such the blessed Baptist, as the forerunner of the Saviour, cried, saying, Prepare ye the way of the Lord, make His paths straight.5
The man therefore was thoroughly an unbeliever, and perverse, refusing the straight paths, straying from the mark, and wandering from the right ways. And Christ deigns not to be with such as are thus minded, and have fallen into this wickedness: and if one may speak in the manner of men, He is tired and weary of them. And this He teaches us saying, How long shall I be with you, and suffer you? For he who says, that those were powerless for the expulsion of evil spirits, who by Christ's will had received power to cast them out, finds fault with the grace itself, rather than with the receivers of it. it was wicked blasphemy therefore: for if grace be powerless, the fault and blame is not theirs who have received it, but rather belongs to the grace itself. For any who will may see that the grace which wrought in them was Christ's. For, for instance, the lame man at the beautiful gate of the temple was made whole; but Peter ascribed the miracle to Christ, saying to the Jews, For Him Whom ye crucified, even by Him this man stands before you whole: and the grace which He bestows hath given him this soundness.6 Elsewhere the same blessed Peter proclaimed to one of those who were healed by Him, Aeneas, Jesus Christ healeth thee.7 It is plain therefore in every way that the man wickedly found fault with Christ's power in saying of the holy apostles, they could not cast it out.8
A similar interpretation of the parallel passage in Matthew can be found in John Chrysostom's (c 349-407) commentary.
The above explanation may seem inconsistent with what Matthew recounts occurring shortly afterward:
Matthew 17:19-21 (RSV)
Then the disciples came to Jesus privately and said, “Why could we not cast it out?” He said to them, “Because of your little faith. For truly, I say to you, if you have faith as a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible to you.”
If anything, however, this actually reinforces the claim that Jesus had not speaking to the Apostles. If he had been, it would have been clear to them that they were the ones at fault for being faithless and perverse, and they would have had no need of asking further. Knowing, however, that they had been given power to cast out demons previously (Matthew 10:8), they remained perplexed. Theophylact's commentary explains here:
The apostles were afraid that they had lost the grace against demons that had been given them; this is why they asked Jesus in private and with great anxiety. But the Lord reproves them for being imperfect in faith, saying, "Because of your unbelief." For if you had fervent, ardent faith, you would accomplish great things even though they appeared to be small.9
Thus, the Apostles may not completely blameless with regard to faith in this episode, but they are not the ones whom Jesus is calling out above as "faithless and perverse."
1. Explanation of the Holy Gospel According to St. Matthew (tr. from Greek; Chrysostom Press, 1992), p.149
2. Sermon LII on Luke (tr. from the Syriac)
3. Proverbs 2:15
4. Psalm 100:4 LXX
5. Matthew 3:3; also Mark 1:3, Luke 3:4, John 1:23
6. Acts 4:10
7. Acts 9:34
8. Op. cit.
9. Op. cit., p.150