Of the testimony of God makes him a liar, and is a sin of the greatest enormity. It is the work of a depraved and guilty heart; for no one without this bias could reject the abundant witness God furnishes of the truth of his word, Psalms 14:1. Especially is unbelief towards an offered Savior an unspeakable crime, justly sealing the condemnation of him who thus refuses to be saved, John 5:18 1 John 5:10.
Source: ATS Bible Dictionary
Unbelief
UNBELIE'F, n.
1. Incredulity; the withholding of belief; as, unbelief is blind.
2. Infidelity; disbelief of divine revelation.
3. In the New Testament, disbelief of the truth of the gospel, rejection of Christ as the Savior of men, and of the doctrines he taught; distrust of God's promises and faithfulness, &c. Matt. 13. Mark 16. Heb. 3. Rom. 4.