George Mueller addresses the destructive nature of unbelief, describing it as the 'white devil' that prevents sinners from approaching Jesus Christ. He explains how unbelief masquerades as wisdom, convincing individuals of their unworthiness and unpreparedness to receive grace. This sin not only undermines faith and prayer but also sows doubt about God's promises and love, leading souls away from God. Mueller emphasizes that unbelief is a subtle and pervasive force that can weaken one's spiritual life and relationship with God.

What an evil it is that keeps sinners from coming to Jesus Christ? And that evil is unbelief: for by faith we come; by unbelief we keep away. Therefore it is that by which a soul is said to depart from God: because it was that which at first caused the world to go off from Him, and that also, that keeps them from His to this day. . . . This sin may be called the white devil. . . In its mischievous doing in the soul, shows as if it were an angel of light: yea, it acts like a counselor of heaven. . . . 1. It is that sin, above all others, that has some show of reason in its attempts. For it keeps the soul from Christ, by pretending its present unfitness and unpreparedness: as want of more sense of sin, want of more repentance, want of more humility, want of a more broken heart. 2. It is the sin that most suits with the conscience. The conscience of the coming sinner tells him, that he has nothing good! . . . that he is a very ignorant, blind and hard-hearted sinner, unworthy to be once taken notice of by Jesus Christ; and will you (says unbelief) in such a case as you re now, presume to come to Jesus Christ? 3. It is the sin that most suits with our sense of feeling. The coming sinner feels the workings of sin, of all manner of sin and wretchedness in his flesh; he also feels the wrath and judgment of God due to sin and ofttimes staggers under it. Now, says unbelief, you may see you have no grace; for that which works in you is corruption. You may also perceive that God does not love you, because the sense of His wrath abides upon you. Therefore, how can you bear the face to come to Jesus Christ? 4. It is that sin above all others that most suits the wisdom of our flesh. . . . And this wisdom unbelief falls in with. 5. It is the sin above all others, that continually is whispering in the ear the soul, with mistrusts of the faithfulness of God, in keeping promise to them that come to Jesus Christ for life. It also mistrusts about Christ's willingness to receive it, and save it. And no in can do this so artfully as unbelief. 6. It is also that sin which is always at hand to enter an objection against this or that promise, that by the Spirit of God is brought to our heart to comfort us. And if the poor coming sinner is not aware of it, it will by some exaction, slight, trick, or cavil, quickly wrest from him the promise again, and he shall have but little benefit of it. 7. It is that above all other sins, that weakens our prayers, our faith, our love, our diligence, our hope and expectations. It even takes the heart away from God in duty. 8. Lastly, this sin . . . even now, appears in the soul with so many sweet pretenses to gather safety and security, that it is, as it were, counsel sent from heaven; bidding the soul be wise, wary, considerate, well advised, and to take heed of too rash a venture upon believing. "Be sure, first, that God loves you; take hold of no promise until you are forced by God unto it; neither be sure of your salvation; doubt it still, though the testimony of the Lord has often been confirmed in you. Live not by faith, but by sense; and when you can neither see nor feel, then fear and mistrust, then doubt and question all." This is the devilish counsel of unbelief, which is so covered over with specious pretenses, that the wisest Christian can hardly shake off these reasonings. . . . (Excerpted from The White Devil by John Bunyan).