Matthew 17:17-21
19. Then came the disciples to Jesus apart, and said, Why could we not 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.
The disciples were confused here. They had been taught by Jesus, and they thought they were ready for any type of spiritual warfare. So when a man brought his demon-possessed son to them for healing, they jumped at the opportunity. However, their efforts were fruitless- they couldn't cast out the demon. So, the man brought his son to Jesus, and he was healed.
When they asked Him why they had no success, Jesus responded that it was because of their unbelief. Imagine their shock! However, there was an important lesson here. Jesus was telling them that the faith they had was not strong enough. They had not done what it took to build and strengthen their faith to where it needed to be, and they had bitten off more than they could chew.
You see, faith is like a muscle. What you can do with it depends on how strong it is. (See Philippians 4:13) Faith needs exercise- Jesus made it clear that strong faith "goeth not out but by prayer and fasting". And as you read these verses, it will shock you how small their faith- and ours- really was. Faith as small as a "grain of mustard seed" can move mountains, and yet how often are we tripped up and defeated by small and seemingly insignificant problems? Talk about being convicted!
Are we willing to do what it takes to build strong faith? If not, then we'll never know true victory in our Christian lives.