"Unbelief deserves as high a place among sins as faith has among graces. Unbelief is the Beelzebub, or prince of sins, which makes others sin. God branded Jeroboam as one "who did sin, and who made Israel to sin" (1 Kings 14:16). Unbelief is a sin-making sin.
The first poisonous breath, which Eve took in from the tempter, was sent in these words: "Now the serpent was more crafty than any other beast of the field that the LORD God had made. He said to the woman, “Did God actually say, ‘You* shall not eat of any tree in the garden’?” " Genesis 3:1. It is as if he had said, "think about this now. Do you really believe God would keep back the best fruit in the whole garden from you?" This was the traitor's gate whereby all other sins rushed into Eve's heart; and even now Satan continues to hold this same gate wide open.
The devil sets up a blind of unbelief between the sinner and God so that he will not fear the Father's warning and chastening aimed at his heart. Then once there is a barricade between him and these merciful bullets, the sinner can be bold with his lust. Unbelief not only diverts the bullets of wrath which are sent out of the law's fiery mouth, but it also retards the actions of grace coming from the Gospel. All the offers of love, which God makes to an unbelieving heart fall like sparks into a river; they are put out as soon as they fall into it.
"For good news came to us just as to them, but the message they heard did not benefit them, because other were not united by faith with those who listened" Hebrews 4:2. The secret of sin's strong hold upon a person is unbelief. There is no mastering a sinner while unbelief overpowers him. This sin will break down all reasoning as easily as Samson did the doors and posts, bar and all, from the city of Gaza. But Samson lay till midnight, and at midnight he arose and took hold of the doors of the gate of the city and the two posts, and pulled them up, bar and all, and put them on his shoulders and carried them to the top of the hill that is in front of Hebron Judges 16:3. It is a sin, which holds out last on the battlefield, the one which the sinner is least aware of, and which the saint ordinarily conquers last. It is one of the chief fortresses to which the devil retreats when other sins are routed.”
Quoted material from, ”The Christian in Complete Armour Daily Readings in Spiritual Warfare” by Gurnall and James S Bell. http://www.moodypublishers.com/pub_productDetail.aspx?id=41823&pid=53617