This is a faculty in man, acquired in the fall (Gen. 3:5-7, 22), whereby he has an inherent, conscious knowledge of good and evil. The word “conscience” appears only once in the Old Testament (Eccl. 10:20 – margin), but the fact of its presence in man is alluded to in a number of places. Proverbs 20:27, for instance, says that it works as “the candle of the Lord searching the inward parts of the belly.” (“Belly” is used as a figure to denote the deepest part of a person’s being.) It is also alluded to as “a still small voice” in man, whereby he knows right and wrong (1 Kings 19:12). Also, when David did something wrong, it says that his heart “smote” him (1 Sam. 24:5; Job 27:6). These things are the effects of the working of the conscience in men.
In the New Testament, the “conscience” is defined as that which bears witness in the thoughts and hearts of men concerning good and evil (Rom. 2:14-15; 7:7). A man is able to condemn another man for his bad conduct, because he has a moral standard in himself (through his conscience) that enables him to judge what is morally right and wrong. In Romans 2:13-15, Paul explains that even though the Gentiles haven’t had the Law of Moses (the Ten Commandments) given to them, they will, if they follow their conscience, “do by nature the things contained in the Law.” This is because the moral standards of God have been written in their hearts. This inward witness is “a law unto themselves.” A person, therefore, does not need a formal law telling him that it is wrong to murder, steal, and commit adultery, etc., before he knows that those things are wrong. The Creator has written on our hearts how we ought to live as responsible, moral beings, and our consciences bear witness to it.
If a man knows his doings are in accord with this inward law, his conscience is “good” (Acts 23:1; 1 Tim. 1:5, 19; Heb. 13:18; 1 Peter 3:16, 21), “pure” (1 Tim. 3:9; 2 Tim. 1:3), and “void of offence” (Acts 24:16). If what he does is not good, his conscience will be “defiled” (Titus 1:15; 1 Cor. 8:7) and “evil” (Heb. 10:22) and it will accuse him of his wrongs. If his life is sustained by evil deeds, his conscience will become “seared” or cauterized (1 Tim. 4:2), and he will become insensible to its accusations. Thus, at that point his conscience is no longer a reliable witness, as seen in those described in Romans 1. However, men with seared consciences are still responsible for what they do.
The gospel brings the conscience of man into the full light of God. It explains the finished work of Christ on the cross, and the person who believes the gospel of his salvation is “sealed” with the Holy Spirit (Eph. 1:13) and is given a “purged” conscience (Heb. 9:14; 10:2). This does not mean that the believer will no longer be conscious of sinning, but as far as the eternal judgment of his sins is concerned, he understands that it has all been taken care of in the atoning work of Christ, and his conscience is forever silenced as to this matter.