And they which heard it, being convicted by their own conscience, went out one by one, beginning at the eldest, even unto the last: and Jesus was left alone, and the woman standing in the midst.
~ John 8:9

Speaking lies in hypocrisy; having their conscience seared with a hot iron;
~ 1 Timothy 4:2

Wherefore ye must needs be subject, not only for wrath, but also for conscience sake.
~ Romans 13:5

Conscience, I say, not thine own, but of the other: for why is my liberty judged of another man’s conscience?
~ 1 Corinthians 10:29

Pray for us: for we trust we have a good conscience, in all things willing to live honestly.
~ Hebrews 13:18

The Nature of Conscience, by Richard Sibbes.

Every man feels and knows what conscience means. There are many rigid disputes of it among the schoolmen…They have much jangling about the description of it, whether it be the soul itself, a faculty, or an act.

In a word, conscience is all these in some sort in diverse respects. Therefore, I will not wrangle with any particular opinion.

1. For what is conscience but the soul itself reflecting upon itself? It is the property of the reasonable soul and the excellence of it that it can return upon itself. The beast…cannot return upon itself and recoil. But the soul of the reasonable creature…knows himself and understands his own excellence. And wheresoever there is understanding, there is an act of reflection whereby the soul returns upon itself and knows what it doth. It knows what it wills; it knows what it affects…It is the property of the soul.

Therefore, the original word in the Old Testament that signifies the heart is taken for the conscience. Conscience and heart are all one. I am persuaded in my soul, that is, in my conscience; and the Spirit witnesseth to our spirit, that is, to our conscience. Conscience is called the spirit, the heart, the soul because it is nothing but the soul reflecting and returning upon itself.

Therefore, it is called conscience, that is, one knowing joined with another because conscience knows itself, and it knows what it knows. It knows what the heart is. It not only knows itself, but it is a knowledge of the heart with God. It is called conscience because it knows with God; for what conscience knows, God knows, which is above conscience. It is a knowledge with God and a knowledge of a man’s self. And so, it may be the soul itself endued with that excellent faculty of reflecting and returning upon itself. Therefore, it judgeth of its own acts because it can return upon itself.

2. Conscience likewise in some sort may be called a faculty. The common stream runs that it is a power. It is not one power, but conscience is in all the powers of the soul; for it is in the understanding, and there it rules. Conscience is that by which it is ruled and guided. Conscience is nothing but an application of it to some particular, to something it knows, to some rules it knows before. Conscience is in the will, in the affections, the joy of conscience, and the peace of conscience, and so it runs through the whole soul. It is not one faculty or two, but it is placed in all the faculties.

3. And some will needs have it an act, a particular act, and not a power. When it doth exercise conscience, it is an act. When it accuseth, excuseth, or when it witnesseth, it is an act. At that time, it is a faculty in act. So that we need not to wrangle whether it be this or that. Let us comprehend as much in our notions as we can: it is the soul, the heart, the spirit of a man returning upon itself; it hath something to do in all the powers; and it is an act itself when it is stirred up to accuse or to excuse; to punish a man with fears and terrors, or to comfort him with joy and the like.

Now conscience is a most excellent thing—it is above reason and sense, for conscience is under God and hath an eye to God always. Therefore, an atheist can have no conscience because he takes away the ground of conscience, which is an eye to God. Conscience looks to God. It is placed as God’s deputy and vicegerent in man. Now it is above reason in this respect. Reason saith (that) you ought to do this: it is a comely thing, it is a thing acceptable with men amongst whom you live and converse, it becomes your condition as you are a man to carry yourself thus, it agrees with the rules and principles of nature in you. Thus saith reason, and they are good motives from reason. But conscience goeth higher. There is a God to Whom I must answer; there is a judgment—therefore I do this, and therefore I do not this. It is a more divine, a more excellent power in man than anything else—than sense or reason or whatsoever. As God planted it for special use, so it looks to God in all.

Therefore, the name for conscience in the Greek and Latin signifies “a knowledge with another” because it is a knowledge with God. “God and my own heart know this. God and my conscience,” as we use to say.

There are three things joined with conscience.

1. It is a knowledge with a rule—a general rule. That is always the foundation of conscience in a man. For there is a general rule: whosoever commits murder, whosoever commits adultery, whosoever is a blasphemer, a swearer, a covetous, corrupt person, “shall not inherit the kingdom of God,” as the apostle saith (1Co 6:9). Here is the general rule. Now conscience applies it: “But I am such a one, therefore I shall not enter heaven.” So, here the conscience practiseth with a rule. It is a knowledge of those particulars with a general rule. And then,

2. It is a knowledge of me—of my own heart. I know what I have done, I know what I do, and in what manner, whether in hypocrisy or sincerity; I know what I think. And then,

3. It is a knowledge with God. For God knows what conscience knows. He knows what is thought or done. Conscience is above me, and God is above conscience. Conscience is above me and above all men in the world, for it is immediately subjugated to God. Conscience knows more than the world, and God knows a thousand times more than conscience or the world. It is a knowledge with a general rule; for where there is no general rule, there is no conscience. To make this a little clearer, all have a rule. Those that have not the Word, which is the best rule of all, yet they have the Word written in their hearts. They have a natural judicature in their souls: “Their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another” (Rom 2:15). They have a general rule: “You must do no wrong; you must do that which is right.”

In the soul, there is a treasure of rules by nature. The Word doth add more rules—the law and the gospel. And that part of the soul that preserves rules is called intellectual because it preserves rules. All men by nature have these graven in the soul. Therefore, the heathen were exact in the rules of justice, in the principles that they had by nature grafted and planted in them.

Now because the copy of the image of God—the law of God written in nature—was much blurred since the Fall, God gave a new copy of His law, which was more exact. Therefore, the Jews, which had the Word of God, should have had more conscience than the heathen because they had a better general rule. And now, having the gospel too, which is a more evangelical rule, we should be more exact in our lives than they.

But every man in the world hath a rule. If men sin without the law, they shall be judged without the law (Rom 2:12) by the principles of nature. If they sin under the gospel, they shall be judged by the Word and gospel. So, conscience is a knowledge with a rule, (a knowledge of) the particular actions that I have done, and a knowledge with God.

In a word, to clear this further concerning the nature of conscience, know that God hath set up a court in man; and there is in man all that are in a court.

1. There is a register to take notice of what we have done. Besides the general rule, for that is the ground and foundation of all, there is conscience, which is a register to set down whatsoever we have done exactly. The conscience keeps diaries. It sets down everything. It is not forgotten, though we think it is, when conscience is once awaked. As in Jeremiah 17:1, “The sin of Judah is written with a pen of iron, and with the point of a diamond” upon their souls. All their wit and craft will not rase it out. It may be forgotten a while by the rage of lusts or one thing or other, but there is a register that writes it down. Conscience is the register.

2. And then there are witnesses: the testimony of conscience. Conscience doth witness, “This I have done, this I have not done.”

3. There is an accuser with the witness. The conscience accuseth or excuseth.

4. Then there is the judge. Conscience is the judge. There it doth judge, “This is well done, this is ill done.”

5. Then there is an executioner. Conscience is that too. Upon accusation and judgment, there is punishment. The first punishment is always within a man before he goes to hell. The punishment of conscience is a pre-judgment of future judgment. There is a flash of hell presently after an ill act. The heathen could observe that God hath framed the heart and the brain so that there is a sympathy between them. Whatsoever is in the understanding that is well and comfortable, the understanding in the brain sends it to the heart and raiseth some comfort. If the understanding apprehends dolorous things, ill matters, then the heart smites, as “David’s heart smote him” (1Sa 24:5). The heart smites with grief for the present and with fear for the time to come.

In good things, a good excusing conscience brings joy presently and hope for the time to come.

God hath set and planted in man this court of conscience; and it is God’s hall, wherein He keeps His first judgment, wherein He keeps His assizes. And conscience doth all the parts. It registereth, it witnesseth, it accuseth, it judgeth, it executes—it doth all.

Besides His love to us to keep us from sin, and then by smiting us to drive us to conversion and repentance to turn from our sins to God, one main end among the rest is to be a pre-judgment, which makes way to God’s eternal judgment; for therein things are judged before. When God lays open the book of conscience, when it is written there by this register, we shall have much to do to excuse ourselves or to plead that we need many witnesses; for our conscience will accuse us. We shall be self-accusers and self-condemners as the apostle saith. Conscience will take God’s part, and God will take part with conscience. And God hath planted it for this main end that He might be justified in the damnation of wicked men at the Day of Judgment.

Now you see, in general, what the nature of conscience is and why it is planted in us by God.

There are four sorts of consciences: some bad and unquiet, some bad and quiet, some good and unquiet, some good and quiet. For a conscience to be bad and quiet is the worst temper that can be! It is better to have a bad unquiet than a bad and quiet conscience; better to have a tormenting (hell) in the soul than a fool’s paradise. The best frame of conscience is the good and quiet conscience. This is a paradise upon earth…a mansion for the Trinity to dwell in.—Edmund Calamy