We don't often consider unbelief to be such a big deal. But J.C. Ryle in his Expository Thoughts on the Gospels points out several truths about this grave wickedness. First, it is the oldest in the world. Adam & Eve's sin in the garden was essentially a sin of unbelief. God had told them that if they ate of the fruit they would die. They didn't believe Him.
Second, it is the most ruinous of all sins because if the consequences of it. Unbelief brought death into the world; unbelief kept Israel out of the promised land for 40 years.
Third, it is the most foolish sin. Unbelief is a choice; a choice that is made in spite of the plainest evidence, in spite of the clear testimony of scripture and the testimony of the Spirit. Ryle says, "It is neither the want of evidence, nor the difficulties of Christian doctrine that make men unbelievers. It is want of will to believe."
Finally, unbelief is the commonest sin. Everyone, believer and nonbeliever alike, wrestle with this sin. Either we commit it with our minds and lips, or we commit it with our lives.
Unbelief is a grave, dangerous sin. Let us guard our hearts against it. Let us, as Ryle exhorts, "...go on watching our hearts, even after we have believed. The root of unbelief is never entirely destroyed."