What is conscience? In his book The Clean Heart, G. H. Lang offers some interesting and helpful insights on this often-misunderstood part of mankind. Here is what he says:
In the human mind there is a divinely implanted monitor named conscience. The word means “joint-witness,” and carries the idea of the man being a witness for or against himself, according as he deems his act good or bad. “Conscience is the consciousness man has of himself in his relation to God, manifesting itself in the form of a self-testimony” (Cremer, 215). It is an activity of the heart, for we read that by nature men “show the work of the law written in their hearts, their conscience bearing witness therewith, and their thoughts one with another accusing or else excusing them” (Rom. 2:15).
Conscience is not infallible; it cannot determine whether an act is good or bad. It is the judgment which determines this, being the judge of the rightness or wrongness of a matter; and the judgment of fallen man being blinded and biased needs the guidance of the Word and Spirit of God in forming decisions.
The function of the conscience is this: If the judgment decides that an act if right, the conscience urges that the thing should be done and reproves the man if he do it not; but if the judgments decides that the thing is wrong, the conscience protests against it being done and reproves the man should he do it. Hence a murderer of the saints of God, and a blasphemer, “verily thought with himself that he ought to do” his persecuting work (Acts 26:9 and compare Prov. 16:2); but later, his judgment being rectified, he as heartily condemned that work (1 Cor. 15:9) Let me therefore be cautious in condemning my neighbor as unconscientious, for one may be doing a wrong act with a good conscience.
If, therefore, today one deems an act right, conscience approves the doing and reproves the not doing; if however tomorrow, through clearer light one has altered his judgment, and deems that wrong which yesterday he deemed right, now conscience reproves the doing and approves the not doing. From this it is clear that no one is justified in doing a thing merely because his conscience may not condemn it, for conscience is only the office of the court who enforces the decisions of the judge, the judgment, that is, the man himself. If the judge alters his decision, conscience as definitely enforces the new verdict as it did the former.
The Clean Heart by G. H. Lang, pp. 31-32