Unbelief

Unbelief is actually perverted faith, for it puts its trust not in the living God but in dying men.
A.W. Tozer

In all unbelief there are these two things; a good opinion of one's self, and a bad opinion of God.
Horatius Bonar

Unbelief is not a misfortune to be pitied; it is a sin to be deplored. Its sinfulness lies in the fact that it contradicts the word of the one true God and thus attributes falsehood to him.
John Stott

Conversion, then, is repentance (turning from sin and unbelief) and faith (trusting in Christ alone for salvation). They are two sides of the same coin. One side is tails -- turn tail on the fruits of unbelief. The other side is heads -- head straight for Jesus and trust his promises. You can't have the one without the other any more than you can face two ways at once, or serve two masters.
John Piper