Unbelief is anything in our heart that doesn't believe God is Good.

Unbelief is any part of our heart that does not believe God is good. Unbelief is wherever the expectation of the negative occurs. When we anticipate negative results we are making room for that experience to occur in our lives. 

Romans 11:22 The Passion Translation (TPT)

22 So fix your gaze on the simultaneous kindness[a] and strict justice[b] of God. How severely he treated those who fell into unbelief! Yet how tender and kind is his relationship with you. So keep on trusting in his kindness; otherwise, you also will be cut off.

Consider therefore the kindness and sternness of God. Kindness to you but sternness to those who fell away (those who did not believe). if you believe in the kindness of God you will experience but if you believe him to be cruel or harsh you are cutting yourself off from the olive tree of the Lord. The olive tree is metaphorical to the grace of God (the empowerment and pleasure of God looking at us face to face and living through us).

Only when you think God is not good you cut yourself off from the grace that is available to you. You cut yourself off from the relationships available to you. You cut yourself off from the presence, power, gifts, character and provision God wants to give you.