​Another passage to consider is Matthew 17:20 - So Jesus said to them, “Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. Jesus made a similar statement in Luke 17:6.
 
Obviously, this is a figure of speech as a mustard seed doesn’t have faith so we’re left to figure it out.  So, my first observation is that unbelief (Greek apistia) and faith (Greek Pistis) are both nouns.  So, Jesus wasn’t referring to how strongly they believe, but rather the content of what they believed.  Faith is based on knowledge Biblical faith on God’s word as explained in Romans 10:17 “so then faith comes by hearing, and hearing by the word of God.”  You can’t believe what you do not know. 
 
So, figuratively what does a mustard seed know?  That it can sprout and grow into a mustard plant.  That is the complete content of its purpose and knowledge.  I believe what the Lord was saying here is simply this, if you know what God wants you to do then you can do it.  The Disciples had been given the power to cast out demons in Chapter 10 when Jesus sent them out to the cities in Israel.  Here in Matthew 17 a man came and asked the disciples to cast a demon out of his son, but they could not.  Why not?
 
Well, this took place on the day after the transfiguration.  Peter, James and John had gone up the mountain with the Lord for that event.  And according to Mark this man brought his son the 9 disciples who stayed behind.  When the father showed up with his son a group of Scribes were in a debate with those disciples.  That may be a factor in their failure to cast out the demon.  Based on Christ’s mustard seed illustration I suspect those scribes had shaken the disciple confidence thus fomenting the failure.  Of course that is just speculation on my part, but Jesus Christ did say their failure was due to their unbelief.  
 
For some reason those 9 disciples could not do what they once had be able and it was due to unbelief.  I’m sure they tried.  I would guess they really wanted to help this man.  But it didn’t require effort or strong desire. It only required they believe the right thing.  My guess is they forgot something Jesus Christ had told them back in chapter 10.

The words belief and faith are nouns while believe is a verb.  The Greek word for faith is Pistis.  It too is a noun just like its English equivalent.  Pistis is also translated as Belief in 2 Thessalonians 2:13 (NKJV).  Being a noun Pistis is used of what is believed, that is the content or object of what is believed.  It could also refer to any collective system of beliefs. 
 
The Greek word usually translated as believe is Pisteuo.  Think of it as the verb form of Pistis.  To believe something means to be persuaded it is true.  Depending on the context it can also be translated as trust.  However, understand that trust is the result of a belief, an action taken because of a belief rather than the act of believing.  Believing precedes trust. 
 
So, let’s apply this to interpreting Scripture.  In Matthew 8:26 Jesus Christ accused the disciples of having little faith.  BTW, He only used that term with His disciples.  The incident that provoked this accusation was a wind storm that came up while He and the disciples were crossing the Sea of Galilee.  
  
As I pointed out in the previous post Jesus Christ never attached an adverb to the word believer (Pisteuo).  He treated as a simple concept.  You either believe something is true or you do not.  There is not almost with believing.  To mostly believe or almost believe means you don’t believe.  The Lord had told them to cross to the other side of the lake in a boat and so they did.  When the wind storm came up and the boat was being tossed about, they became afraid and pleaded with Christ to save them.  That’s when He said “Why are you fearful, O you of little faith?” 
 
The question itself indicates they really had no reason to be afraid.  It was because their faith was small, it lacked sufficient content.  Remember faith is about what you believe.  They forgot that Jesus being God could say with certainty they would cross the lake.  They were not across yet so no matter how high the waves got they were still safe.   Little Faith is actually one word in the Greek, Oligolpistos.  Oligos is a volumetric term meaning small, short or few and Pistis means faith.  So, Jesus was commenting on the size or volume of their faith and not on how strongly they held it.  They were not believing everything Jesus had said.