17 And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name.
18 And he said unto them, I beheld Satan as lightning fall from heaven.
19 Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you.
20 Notwithstanding in this rejoice not, that the spirits are subject unto you; but rather rejoice, because your names are written in heaven.
14 And when they were come to the multitude, there came to him a certain man, kneeling down to him, and saying,
15 Lord, have mercy on my son: for he is lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water.
16 And I brought him to thy disciples, and they could not cure him.
17 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.
18 And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.
19 Then came the disciples to Jesus apart, and said, why could not we cast him out?
20 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.
21 Howbeit this kind goeth not out but by prayer and fasting.
Up until this time the disciples had great success in casting out devils, but they couldn't cast out this devil. Jesus plainly tells the disciples that their failure to cast out this particular demon had to do with "THEIR UNBELIEF"
This proves beyond a doubt that faith is a spiritual force that must come out of your spirit. No doubt the disciples followed the same routine they had followed countless times before as they cast out many demons. Speaking to them and commanding them to come out, but this time it didn't work.
They obviously THOUGHT they could cast out this demon. We see that mental agreement does not get the job done, it is only one small step in the process. You can use the same thought process as it applies to other aspects of using faith. The same principle applies to everything in this life that we need faith to procure or cause to happen.
The bottom line is that if you are not getting results then enough faith is not being generated to get the job done. We must do the things in life that is conducive to building faith.
Jesus gives us one such clue in this passage of scripture. VS 21 says that we can also use fasting and prayer to help build up our faith.
Edward E. Hill