Unbelief is not simply an infirmity of fallen human nature
"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