Because of Your Unbelief
It is recorded in Matthew 17:14-20 that a man asked Jesus for help because His disciples were not able to cast the demons out of his son. After Jesus cast out the unclean spirits, His disciples came to ask Him why they were not able to do the same, and Jesus answered:
Because of your unbelief. For verily I say to you, if you have faith as a grain of mustard seed, you shall say to this mountain, "Move from here to there", and it will move; and nothing will be impossible for you. Matthew 17:20
What is the Faith that Moves Mountains?
Jesus said, if we have (strong) faith, we can move mountains, and nothing will be impossible to us. Who doesn't want this kind of faith? But how come most people do not experience it? Jesus then went on to explain what this "faith" is like.
1. Faith is NO Unbelief (Doubt) At All
Jesus said the reason His disciples could not cast out the demons is because of their "unbelief". In other words, if they had no "unbelief" (doubt) at all, they should have been able to expel the demons. But what does it mean by "no doubt at all"? We may be able to gain a better understanding by looking at the behaviors of prophet Samuel.
When the Lord commanded prophet Samuel to anoint another King to replace Saul, he went to the house of Jesse. Jesse then asked his seven sons to pass before Samuel, but he could not find the one that the Lord had chosen. So what did Samuel do? He questioned the father! "Are all your sons here?" (1 Samuel 16:11), he asked. I find this so remarkable because who would have doubted the father for not knowing the number of his sons? However, Samuel immediately doubted Jesse because what he said was contrary to what the Lord had told him. Since it is impossible for the Lord to be wrong, so it must be Jesse! Samuel had NO unbelief at all! And that was probably why everything he said came to pass!
And Samuel grew, and the LORD was with him, and did let none of his words fall to the ground. 1 Samuel 3:19
2. Faith is Like a Mustard Seed
What does it mean by having faith like a mustard seed? Jesus explained this in Mark 4:31-32.
It (Faith) is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth. But when it is sown, it grows up, and becomes greater than all herbs, and shoots out great branches; so that the fowls of the air may lodge under the shadow of it. Mark 4:31-32
Mustard seed has three characteristics. One, it is the smallest of all seeds; Two, it needs to grow and develop; Three, it becomes the largest when it is fully grown, and it protects all the lives that lodge under it.
Likewise, FAITH IN GOD starts out to be the smallest (most people believe in doctors more than divine healing). It needs to be developed, but when it is fully grown, it has the greatest power of all!
This is probably why when Jesus' disciples asked Him why they could not cast out the demons, He replied, "this kind does not go out except by prayer and fasting" (Matthew 17:21). What does prayer and fasting have to do with casting out demons? This is because some demons are particularly cunning and deceitful, like those in this incident which tormented the victim to scare the disciples. His disciples will need much more faith to cast them out, and this can only be developed by prayer and fasting.
Faith of Peter
Importance of FAITH can also be seen in another Bible character Peter, the disciple of Jesus. When Peter saw Jesus walking on water, he was so astonished that he forgot all his unbeliefs. He followed Jesus' command and started to walk on water. However, when doubt came in, he sank immediately.
And Peter answered Him and said, “Lord, if it is You, command me to come to You on the water.” So He said, “Come.” And when Peter had come down out of the boat, he walked on the water to go to Jesus. But when he saw that the wind was boisterous, he was afraid; and beginning to sink he cried out, saying, “Lord, save me!” And immediately Jesus stretched out His hand and caught him, and said to him, “O you of little faith, why did you doubt?” Matthew 14:28-31
Even though Peter's faith started out small like a mustard seed, it grew quickly after the Pentecost, and he healed many sick people, cast out demons and performed numerous miracles. This is a clear example on how one can develop his faith through practicing the Word and following the guidance of the Holy Spirit.