The Greek word for unbelief here is APISTIA (a-pe-ste-a), but if we take it in context it is not unbelief of who God is nor His power--These same disciples had been casting out demons and healing the sick... this unbelief is more of “doubt” that comes from not being sensitive to God’s ways--having a hardened heart.
We don’t like to have someone tell us that we have a hardened heart, because most of us understand that as being against God, and it is--But we all have hardened hearts, unless by the Word of God we have become sensitive to His Word and His ways, and so experienced this for ourselves. Jesus said that it takes faith the size of a mustard seed to move a mountain from one place to another--how much faith do we have? He even goes on to say, “nothing will be impossible for you”--Do we feel that way? Jesus says we have hardened hearts... a heart of unbelief.
How do we remedy that? By the Word of God. By getting to KNOW God well enough, that we can trust in Him in all things. I would imagine when this lunatic boy fell to the grown with convulsions, it took their eyes off of Jesus words and unto the storm; just like it did Peter when he was walking on the water. When we become so sensitive to God, that we see nothing but Him and His Words we have then softened our heart in that area enough to receive in the faith of Christ that can bring about any promise believed for.
Father, thank You for Your Word. May I continue meditating in Your Word until my heart is so softened (sensitive) to Your every Word that I receive victory in all areas of my life where it is needed. I know it is your word that chases every bit of unbelief (doubt) far from me. Lord, I want to be soft on You. Thank you for the privilege of being able to study your word. Amen