(excerpt from The Vanishing Conscience by John MacArthur)
No sin is more destructive to the conscience than the sin that takes place in the arena of the mind. Sins of the mind assault the conscience like no other sins, because the conscience is their only deterrent. After all, who but God and the sinner ever knows bout them? “Who among men knows the thoughts of a man except the spirit of the man, which is in him?” (2 Corin 2:11). Many people who will not do evil deeds are nevertheless boldly evil in their thoughts. A man who abstains from fornication for fear of getting caught might convince himself it is all right to indulge in salacious fantasies because he thinks no one else will ever discover such a private sin. The sins he deliberately entertains in his mind may be a thousand times more evil than anything he would ever think of doing before others. Scripture says his guilt is the same as if he acted out his fantasies.
To indulge in sins of thought, therefore, is to molest the conscience directly. Those whose thoughts are impure cannot have pure consciences; the guilt is inherent in the evil thought. When the thoughts are defiled, the conscience immediately is, too. That is why nothing is more characteristic of unbelief than an impure mind combined with a defiled conscience: “to the pure, all things are pure; but to those who are defiled and unbelieving, nothing is pure, but both their mind and their conscience are defiled” (Tit 1:15) In fact, nothing damages the conscience more than the habit of indulging in evil thoughts. Unfortunately, once begun, the practice becomes all too easy. This is a sin that does not have to wait for an opportunity; the mind can sin anytime, anywhere, under any circumstances. So the habit is quickly and easily established.
By engaging the inner faculties – mind, emotions, desire, memory, and imagination – thought-sins work directly on the soul to bias it towards evil. Sow a thought, reap an act. Sow an act, reap a habit. Sow a habit, reap a character. Sow a character, reap a destiny. Evil thoughts thus underlie and lay the groundwork for all other sins.
No one ever “falls” into adultery. The adulterer’s heart is always shaped and prepared by lustful thoughts before the actual deed occurs. Likewise, the heart of the thief is bent by covetousness. And murder is the product of anger and hatred. All sin is first incubated in the mind.
Jesus taught this truth to His disciples: “The things that proceed out of the mouth come from the heart, and those defile the man. For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. These are the things which defile the man; but to eat with unwashed hands does not defile the man” (Matt 15:18-19)
Watch over your heart
It is relatively easy to confess and forsake deeds of sin, sins of omission, and unintentional sin. But the sins of our thought life are soul-coloring sins, character-damaging sins. Because they work so directly against the conscience and will, dealing with them honestly and thoroughly is one of the most difficult aspects of mortifying our sin. If we ever want to see real progress in sanctification, however, this is an area where we must attack and destroy our sinful habits with a vengeance. If we allow our thoughts to be influenced by the values of the world, our conscience will surely be dulled. Listening to and entertaining the claims of bad theologies of the self-esteem credo of modern psychology will surely deaden the conscience. Not only thoughts about lust, envy, and other traditional sins, but also thoughts about the myriad false values and idols of an unbelieving world can be devastating obstacles to a pure mind.
The Old Testament sage wrote, “Watch over your heart with all diligence, for from it flow the springs of life” (Prove. 4:23)
God knows our hearts (Acts 15:8). “God is greater than our heart, and knows all things” (1 John 3:20). David wrote, “Thou dost understand my thought from afar…and art intimately acquainted with all my ways. Even before there is a word on my tongue, behold, O Lord, Thou dost know it all” (Ps 139:2-4). Why, then, would we ever feel free to indulge in gross sins in our imagination – sins we would never act out before others – when we know that God is the audience to our thoughts? “Would not God find this out? For He knows the secrets of the heart” (Ps 44:21).