“Truth is a strong tower and never requires to be buttressed with error. God’s Word will stand against all man’s devices.

I would never invent a sophism to prove that it is no sin on the part of the ungodly not to believe, for I am sure it is, when I am taught in the Scriptures that, ‘This is the condemnation, that light is come into the world, and men love darkness rather than light,’ and when I read, ‘He that believeth not is condemned already, because he believeth not on the Son of God,’ I affirm, and the Word declares it, unbelief is a sin.

Surely with rational and unprejudiced persons, it cannot require any reasoning to prove it. 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.”

- CH Spurgeon, “The Sin of Unbelief”