Why we couldn't drive it out?

This was the question of the 12 disciples thrown to Jesus after they have tried to drive out the demons inside the body of a boy. lets take a look at the verse


Matt. 17:16 "I brought him to your disciples, but they couldn't heal him.", what a shame. But why does the disciple of Christ couldn't drive it out?


Matt. 17:20-21 "If you have a faith as small as a mustard seed, you can say to this mountain, 'Move from here to there and it will move', Nothing will be impossible to you."


2 things I have learned from this passage.


First, you don' need to have a very powerful aura to drive demons or to do the impossible things. You don't have to be superman to do it. In doing it, it just require of one thing. FAITH.


Secondly, faith is requirement to do the impossible. Doesn't matter if you have a small faith or large faith but as long as you have it, then you probably do it. 


But where should our faith came from? Romans 10:17 "comes from hearing and hearing the message through the word of Christ.", this speak about our devotional and obedience to Christ.


Why we couldn't drive it out?because of no faith.