Matthew 17:20

And he said unto them, Because of your little faith: 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.

The reasons for the apostles' failure were (1) their lack of faith, (2) the double difficulty of the case at hand, and (3) their failure to exercise the privilege of prayer and fasting. At this place in Matthew, some very ancient authorities include Matthew 17:21 which reads, "But this kind goeth not out save by prayer and fasting." Mark 9:29 reads, "This kind can come out by nothing, save by prayer." The disciples had some faith, else they would not have tried to cast it out. The fact that they had previously cast out demons but could not cast out that one shows that some demons are more malevolent and stubborn than others. This opens a whole field of questions regarding the character and variety of demons, but the Scriptures afford little information on such a point. The necessity for prayer (certainly) and fasting (perhaps) was stressed. The child had long been possessed by the demon, and the usual pattern of demonic destruction was evident in the danger incurred from falls into the fire and into the water. It is noteworthy that Satan's purpose, wherever revealed in Scripture, invariably appears destructive. In the cases of Job (Job 1:16), Judas (Luke 22:3), the swine (Matthew 8:32), and in many others, death and destruction always resulted quickly when Satan or his emissaries had a free hand to work their will.

Nothing shall be impossible unto you, is a very strong statement by the Lord. One is tempted to make our Lord's remark about removing mountains mere hyperbole, but no such restriction seems justified from the text. To the true believer, and especially to the apostles, all things were possible through faith. To every true child of God, all moral and material difficulties vanish. The tragedy is that most disciples, like the nine in the case here, are hindered by seeds of doubt and unbelief, and perhaps also by the lack of fervent and devoted prayer.