Got this from my inbox today, from CQOD (Christian Quotation Of the Day)
Unbelief is actually perverted faith, for it puts its trust, not in the living God but in dying men. The unbeliever denies the self-sufficiency of God and usurps attributes that are not his. This dual sin dishonors God and ultimately destroys the soul of man.
... A. W. Tozer (1897-1963), The Knowledge of the Holy
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.
-- Hebrews 3:12 (ESV)