Unbelief is not simply an infirmity of fallen human nature

(Arthur Pink)

"The one who does not believe God has made Him a liar, because he has not believed in the testimony God has given about His Son." 1 John 5:10

Unbelief is not simply an infirmity of fallen human nature, it is a heinous crime against God! Scripture everywhere attributes it to . . .
  love of sin,
  obstinacy of will,
  hardness of heart.

Unbelief has its root in a depraved nature - in a mind which is enmity against God.

"He who does not believe the Son shall not see life, but the wrath of God abides on him." John 3:36