Is it not a sin for a creature to doubt the word of its Maker? Is it not a crime and an insult to the Divinity, for me, an atom, a particle of dust, to dare to deny his words? Is it not the very summit of arrogance and extremity of pride for a son of Adam to say, even in his heart, “God I doubt thy grace; God I doubt thy love; God I doubt thy power?” Oh! sirs believe me, could ye roll all sins into one mass,–could you take murder, and blasphemy, and lust, adultery, and fornication, and everything that is vile and unite them all into one vast globe of black corruption, they would not equal even then the sin of unbelief. This is the monarch sin, the quintessence of guilt; the mixture of the venom of all crimes; the dregs of the wine of Gomorrah; it is the A1 sin, the master-piece of Satan, the chief work of the devil.
Spurgeon’s Sermons (Spokane, Washington; Olive Tree Bible Software; 2010) eBook. Vol. 1, Sermon No. 3; Titled: The Sin of Unbelief; Delivered on Sabbath Morning, January 14, 1855.