The books I've been reading lately keep driving home the magnitude of the sin of unbelief. Here's Thomas Watson:
Of all sins, beware of the rock of unbelief. "Take heed lest there be in any of you an evil heart of unbelief" (Hebrews 3:12). Men think, as long as they are not drunkards or swearers, it is no great matter to be unbelievers. This is the gospel sin, it dyes your other sins in grain.
(1) Unbelief is a Christ-reproaching sin. It disparages Christ's infinite merit as if it could not save; it makes the wound of sin to be broader than the plaister [bandage] of Christ's blood. This is a high contempt offered to Christ, and is a deeper spear than that which the Jews thrust into his side.
(2) Unbelief is an ungrateful sin. Ingratitude is a prodigy of wickedness; and unbelief is being ungrateful for the richest mercy. Suppose a king, to redeem a captive, should part with his crown of gold, and when he had done this should say to the redeemed man, "All I desire of you in [repsonse to] my kindness, is to believe that I love you." If he should say, "No, I do not believe any such thing, or that you care at all for me"; I appeal to you whether this would not be odious ingratitude? So is the case here. God has sent his Son to shed his blood; he requires us only to believe in him, that he is able and willing to save us. No, says unbelief, his blood was not shed for me, I cannot persuade myself that Christ has any purpose of love to me. Is not this horrid ingratitude? This enhances a sin, and makes it of a crimson color.
(3) Unbelief is a leading sin. It is the breeder of sin. Unbelief is a root sin, and the devil labours to water this root, that the branches may be fruitful. It breeds hardness of heart; therefore they are put together (Mark 16:14). Christ upbraids [reprimands] them with unbelief and hardness of heart. Unbelief breeds the stone of the heart. He who believes not in Christ, is not affected with his sufferings, he melts not in tears of love. Unbelief freezes the heart; first it defiles and then hardens. Unbelief breeds profaneness. Anunbeliever will stick at no sin; neither at false weights, nor false oaths. He will swallow down treason. Judas was first an unbeliever, and then a traitor (John 6:64). He who ha no faithin his heart, will have no fear of God before his eyes.
(4) Unbelief is a wrath-procuring sin. "He that believeth not on the Son of God, the wrath of God abideth on him" (John 3:36). He who believes not in the blood of the Lamb, must feel the wrath of the Lamb. The Gentiles that believe not in Christ will be damned as well as the Jews who blaspheme him. And if unbelief be so fearful and damnable a sin, shall we not be afraid to live in it? (The Ten Commandments [1692; Banner of Truth, 2009], 201–202)