Unbelief is one of the tools that devil use to steel away the victory of some people.
Unbelief is a killer of smooth relationship between us and God.
He that doubt God stops Him from proving himself great.
Unbelief is one of the causes of people looking for solutions outside God which at the end of days lead to catastrophic.
He that did not believe did not know God and cannot please Him, because "he that come to him must believe that He's God and a rewarder of them that diligently seek Him."