the Bible interprets itself:
Mar 9:14-29 KJV
(14) And when he came to his disciples, he saw a great multitude about them, and the scribes questioning with them.
(15) And straightway all the people, when they beheld him, were greatly amazed, and running to him saluted him.
(16) And he asked the scribes, What question ye with them?
(17) And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit;
(18) And wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not.
(19) He answereth him, and saith, O faithless generation, how long shall I be with you? how long shall I suffer you? bring him unto me.
(20) And they brought him unto him: and when he saw him, straightway the spirit tare him; and he fell on the ground, and wallowed foaming.
(21) And he asked his father, How long is it ago since this came unto him? And he said, Of a child.
(22) And ofttimes it hath cast him into the fire, and into the waters, to destroy him: but if thou canst do any thing, have compassion on us, and help us.
(23) Jesus said unto him, If thou canst believe, all things are possible to him that believeth.
(24) And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.
(25) When Jesus saw that the people came running together, he rebuked the foul spirit, saying unto him, Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him.
(26) And the spirit cried, and rent him sore, and came out of him: and he was as one dead; insomuch that many said, He is dead.
(27) But Jesus took him by the hand, and lifted him up; and he arose.
(28) And when he was come into the house, his disciples asked him privately, Why could not we cast him out?
(29) And he said unto them, This kind can come forth by nothing, but by prayer and fasting.
Mat 17:14-21 KJV
(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 <of unbelief> goeth not out but by prayer and fasting.
read separately, these two reports from different gospel accounts of the healing of the lunatic son seem to disagree; but when merged as different perspectives of the same account, they form a complete picture of how faith works.
Jesus clearly said that if the father believed, then his son would be healed; the father recognized that he believed in his heart, but after seeing the lunacy of his son's actions with his physical eyes, he struggled to believe in his mind - he was 'double minded'
Jas 1:5-8 KJV
(5) If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.
(6) But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.
(7) For let not that man think that he shall receive any thing of the Lord.
(8) A double minded man is unstable in all his ways.
notice that James tells us that a double-minded man will not receive from God; not that God will not give to a double-minded man...
getting back to the account of the lunatic son, the disciples ask Jesus why they could not heal the boy, and Jesus replies 'because of your unbelief'. the disciples had also seen the boy's violent lunatic display with their physical eyes - and had also become double minded. so we see that there is a balance between heart-faith and mental unbelief resulting from input from the physical senses; if the mind focuses on the input from the physical senses, this produces mental knowledge that opposes mental belief in God's power, binding the mustard-seed sized amount of heart-faith we've been given. regular prayer, or focusing the mind on God's promises that overcome the physical world through fellowship with Him, helps to overcome the mental influence that the input from the physical senses can cause - and fasting disciplines the mind to ignore the input from the physical senses that oppose God's promises.
so while faith never varies (which is great news since no one can force God to give them more faith), faith-binding mental unbelief does vary, and is under our exclusive control