Matthew 17:16-20

Mat 17:16 (KJB)
And I brought him to thy disciples, and they could not cure him.

Then the man states that he had already tried bringing his son to the disciples which had stayed behind while the others were up on Mount Meron with Jesus. For some reason the disciples could not free the boy from the devils. (Mat 10:1 KJV) And when he had called unto him his twelve disciples, he gave them power against unclean spirits, to cast them out, and to heal all manner of sickness and all manner of disease. Previously they had been given the authority to cast out devils from people but this time they seemed powerless to be able to do this.

Mat 17:17 (KJB)
Then Jesus answered and said, O faithless and perverse generation, how long shall I be with you? how long shall I suffer you? bring him hither to me.

Perverse - Turned away or corrupted

This was not the first time the people of Israel were accused of being perverse. It came from Moses - (Deu 32:5 KJV) They have corrupted themselves, their spot is not the spot of his children: they are a perverse and crooked generation. Then God Himself made that accusation - (Deu 32:20 KJV) And he said, I will hide my face from them, I will see what their end shall be: for they are a very froward generation, children in whom is no faith. The nation of Israel had seen the miracles of God in time of Moses and yet they were still hardened. They had also been witness to the miracles of Jesus and witnessed the casting out of devils by Jesus and yet this people had remained faithless. Then Jesus asks two questions in a seemingly perturbed way. How long shall He be with them and How long will He bear their unbelief? His earthly ministry was coming to a close and no longer would He be able to instruct the disciples. He also knew that they saw His power over the Kingdom of Satan and how He gave them the powers to cast out the devils and yet they could not. Then He asks them to bring the child to them.

Mat 17:18 (KJB)
And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.

As soon as the child was brought to Jesus, He rebuked the devil and the devil had to depart out of him. These devils had no choice but to obey Jesus because before their rebellion with Satan, they knew exactly who He was. Once the devil left the child, he was instantly cured from that very hour. The words of Jesus are very powerful.

Mat 17:19 (KJB)
Then came the disciples to Jesus apart, and said, Why could not we cast him out?

The disciples did not make any attempt to hide the fact that they could not cast out this devil but they had asked Jesus as to why they were unable to cast out this devil. It sounded like they had previous success in casting out devils because here it almost sounds like a surprise why they could not.

Mat 17:20 (KJB)
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.

Jesus makes a very important statement that they could not cast out the devil because of their unbelief. They may have become frightened at the sight of the child and a person in fear is not exercising faith. When fear builds, faith recedes, and vice versa. Remember the mustard seed is about 1mm large and the mustard plant which grows from it, grows to height of 600 times the size of the seed. So the Lord is telling them that even faith, if it is the size of a mustard seed and if that faith is upon the Lord Jesus Christ, then you will be able to move mountains with it. In fact, nothing shall be impossible for the Christian. Unfortunately, too much faith is diluted by fear and that removes the effectiveness of faith. When Peter had walked upon the water and had his eyes on Jesus, he had the faith that he would be fine but as soon as he started looking at the situation of the storm around him, his faith failed because fear took over. Fear and faith cannot occupy the same space at the same time.