Matthew 17:11-15

Mat 17:11 (KJB)
And Jesus answered and said unto them, Elias truly shall first come, and restore all things.

Jesus did confirm the teaching of the scribes that Elijah was to come and restore all things. The restoration was not one of political nature but of a spiritual and moral nature. (Mal 4:6 KJV) And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse. The disciples, like the others in Israel may have been looking for a physical Messiah to throw off the yoke of Rome. Johnís ministry was a mending ministry of repentance.

Mat 17:12 (KJB)
But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.

Then Jesus confirms to them that Elijah had already come and they did not know him. He was not speaking of a return of the actual prophet but He was speaking about the ministry of John the Baptist. The disciples knew that Herod had him thrown in prison because he pronounced his marriage as sinful and eventually John the Baptist was beheaded for his unrelenting ministry. Then Jesus tells the disciples that just as John had suffered at the hands of these evil leaders, He will also suffer the same fate as He will be crucified.

Mat 17:13 (KJB)
Then the disciples understood that he spake unto them of John the Baptist.

Then the disciples understood that the prophecy of Malachi 4:5 was not speaking of the actual prophet Elijah but was a prophecy of the ministry of John the Baptist. The disciples, having understood what Jesus told them, meant they were satisfied with the answer which Jesus gave them.

Mat 17:14 (KJB)
And when they were come to the multitude, there came to him a certain man, kneeling down to him, and saying,

Then Jesus and Peter, James, and John had now returned to where the other disciples were and encountered a multitude. Then a man who had waited for Jesus comes up to Him and kneels down in front of Him. This crowd gives evidence that this took place in the Upper Galilee near Mount Meron because there have been ruins of ancient synagogues found there and multiple synagogues means a large population.

Mat 17:15 (KJB)
Lord, have mercy on my son: for he is a lunatic, and sore vexed: for ofttimes he falleth into the fire, and oft into the water.

Lunatic - Devil possessed, or one who is moonstruck
Vexed - Experience suffering or affliction

This man comes to Jesus begging mercy on his son because he is as an epileptic but it is more than that disease for he is possessed by devils. This boy was being torn about by the devil, one time he falls into the fire and then he falls into the water. This type of prolonged affliction would eventually lead to his death. The devil which possessed him was probably trying to kill him by what he was putting this child through. The child was actually being tortured. Satanís kingdom is filled with those who desire to bring death to humans.