Unbelief

"Take heed, brethren, lest there be in any of you, an evil heart of unbelief, in departing from the living God." Hebrews 3:12

Unbelief is the prince of sins. As faith is the radical grace, so is unbelief a radical sin,—a sinning sin. As, of all sinners, those are most infamous, who are ring-leaders and make others sin, which is the brand which God has set upon Jeroboam's name, "Jeroboam the son of Nebat, who, sinned, and made Israel to, sin" (1 Kings 14:16), so among sins they are most horrid that are most productive of others, such is unbelief above any other: it is a ring-leading sin, a sin-making sin. The first poisonous breath which Eve sucked in from the tempter, was sent in these words, "Yea, hath God said, Ye shall not eat of every tree of the garden?" (Gen 3:1). As if he had said, Consider well on the matter; do you believe God meant so? Can you think so ill of God, as to believe he would keep the best fruit of the whole garden from you? This was the traitor's gate at which all other sins entered into her heart; and it continues to this day of the same use to Satan, for the hurrying souls into other sins, called therefore "an evil heart of unbelief in departing from the living God." The devil sets up this sin of unbelief, as a blind between the sinner and God; that the shot which come from the threatening, levelled at the sinner's breast, may not be dreaded by him; and then the wretch can be as bold with his lust as the pioneer is at his work, when he has got his basket of earth between him and the enemies' bullets: nay, this unbelief does not only choke the bullets of wrath which are sent out of the law's fiery mouth, but it damps the motions of grace which come from the gospel; all the offers of love which God makes to an unbelieving heart, they fall like seed into dead earth, or like sparks into a river, they pare out as soon as they fall in.

[William Gurnall, Departing from the Living God]