No sin startles less—or damns surer!

(John Flavel "The Method of Grace")

"He who believes on the Son has everlasting life; but
 he who does not believe in the Son shall not see life;
 but the wrath of God abides on him." John 3:36

Unbelief is man's great sin, and condemnation is
his great misery. How dreadful a sin is the sin of
unbelief, which brings men under the condemnation
of the great God. No sin startles less—or damns
surer! Unbelief is a sin which does not affright the
conscience as some other sins do; but it kills the soul
more certainly than any of those sins. Other sins could
not damn us were it not for unbelief, which fixes the
guilt of them all upon us. Unbelief is the sin of sins;
and when the Spirit comes to convince men of sin,
He begins with this as the capital sin.