Proverbs 24:9

The thought of foolishness is sin: and the scorner is an abomination to men.

The thought of foolishness is itself sin, so wrongdoing is not limited to outward acts and words but includes the mind. God's law reaches the thoughts, desires, motives and inclinations of the heart, and falling short of that perfect standard in thought is sin just as in action. A wicked thought that merely arises is not yet sin, but it becomes sin when it is welcomed, retained and entertained in the heart. Thoughts must therefore be guarded and barred against sin as strictly as acts and words, with the first sinful thought checked at once by turning to God in earnest prayer and continually rejecting evil. Even thought kept entirely hidden from others is recorded in heaven and must be confessed to God and forsaken, while saving faith purifies the heart and makes it hate vain and foolish thoughts.

Foolish Thought Is Sin

Even a foolish thought is sin, so sin includes wrongdoing in the mind and not only outward acts and words. Sin can enter the mind before it is expressed, and harboring foolish, unprofitable thoughts is itself sin. Foolish thoughts are not harmless merely because they remain thoughts and are never expressed. They are sin even when they seem harmless or too small to matter, and thoughts that transgress God's law, like other such acts, cannot be dismissed as insignificant.

A temptation or wicked thought that arises is not sin simply because it occurs; it becomes sin when it is welcomed, retained, and entertained. Sin enters unexpressed, invisible thoughts held in council in the bosom, so thoughts must be guarded and barred against sin just as acts and words are. Entertaining folly in the mind violates God's law and belongs among sins of commission.

God's law reaches the thoughts, desires, motives, and inclinations of the heart and must be kept inwardly as well as outwardly. God's perfect law is the standard and expression of His character, so to fall short of it in thought or action is to sin, and sin reaches even the inward motions that may lead to an outward deed. The mind alone can violate God's law, for foolish or wrong thoughts are sin in themselves and not merely causes of later sinful words or actions: hatred of a brother is murder, lustful looking is adultery in the heart, and covetousness is an inward sin.

Guarding and Cleansing Heart

Sin is not confined to outward acts and words but enters unexpressed invisible thoughts held in council in the bosom, so thoughts must be guarded and barred against sin just as acts and words are. Foolish or wrong thoughts are sin in themselves and not merely causes of later sinful words or actions, for the mind alone can violate divine law as when hatred of a brother is murder, lustful looking is adultery in the heart, and covetousness is inward sin, and therefore thoughts must be brought under Christ's control. Entertaining a foolish or evil thought is itself sin, so the first sinful thought must be checked at once rather than allowed to lead toward sinful desire or action. Resisting it is a duty performed by turning immediately to God in earnest prayer for deliverance, remaining vigilant, and continually rejecting evil.

A thought kept entirely hidden from other people is still sin and is recorded in heaven. Such secret sin must be confessed to God and forsaken in order to be covered, for otherwise it will ultimately be revealed. Even the thought of wickedness is sin and not only its outward expression, and wickedness has no fellowship with God but causes His love and favor to withdraw from the human heart. Sin must therefore be uprooted from the heart itself, including the love of sin, before God's love can remain there.

Even the thought of foolishness is sin, and saving faith purifies the heart and produces hatred of sin, vain thoughts, and foolish thoughts. That faith hates the thought of foolishness more than unbelief hates the foolish act.