'You of Little Faith' vs 'Faith as small as a Mustard Seed'

I was recently reading the story of Jesus healing the demon possessed boy. The disciples tried and failed to cast out the demon first, and after Jesus successfully does so, the disciples asked why they couldn't do it. Jesus’ reply in Matthew 17:20 (in the NIV) was as follows:

“Because you have so little faith. Truly I tell you, if you have 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 for you.”

I had originally read this scripture in the NKJV (enjoying that one at the moment)…but the NKJV says “Because of your unbelief” compared to “Because you have so little faith” in the NIV.  So I went back to the Greek and found the word is ‘oligopistian’ which is literally ‘little or low quality faith’…which implies the disciples had SOME faith, but just not enough to do the job…but wait…Jesus goes on to say “if you have faith as small as a mustard seed… nothing is impossible for you”.  (The mustard seed being tiny, Jesus is implying that you only need a little faith)  So what’s the difference?  Was Jesus splitting hairs?? Did the disciples have faith, but just not quite in the amount the size of a mustard seed?!   Of course this seems ridiculous…so what’s the difference between the disciples ‘little faith’ and what Jesus calls ‘faith the size of a mustard seed’?

This is what I found (sticking with the NIV):

In regards to ‘little faith’ - Matthew 14:31 - when Peter walks on water and then begins to sink – “You of little faith (same word as in Matthew 17:20),” he said, “why did you doubt”

In regards to ‘faith the size of a mustard seed’ - Mark 11:23 - “Truly I tell you, if anyone says to this mountain, ‘Go throw yourself into the sea,’ and does not doubt in their heart but believes what they say will happen, it will be done for them.” (see also Matthew 21:21)

So what’s the difference? Doubt.

When we have faith in God that does not doubt, it is so potent that even a tiny little bit (the size of a mustard seed) is enough to do the impossible.

Next question…how do we have faith that doesn't doubt? It seems to be that it’s not just as simple as deciding you are not going to doubt (I tried).

One thing I found in John 20:27 in reference to 'doubting Thomas'

"Then he said to Thomas, 'Put your finger here; see my hands.  Reach out your hand and put it into my side. Stop doubting and believe.'"

It is interesting to note that when Jesus tells Thomas to put his finger in Jesus' hand, the verb is in the present imperative.  The present imperative in Greek is described as a call to a long term commitment.  Jesus is not just asking Thomas to touch his hand once...he is in fact commanding him to a way of life - to a continual action.  And once Thomas touches his hand, Jesus commands Thomas to a greater intimacy of placing his hand in Jesus' side...again an habitual action.  After these two commands, Jesus says to Thomas "stop doubting and believe."

It is when we are actively abiding as a way of life, in the one who gives life, that we then have the ability to stop doubting and believe...