The upright does nothing that is contrary to duty, even though there is a hope of keeping it secret. To point this out the tale about the ring of Gyges was invented by the philosophers. Exposing this, he brings forward known and true examples from the life of David and John the Baptist.
29. To lay down here already the result of our discussion, as though we had already ended it, we declare it a fixed rule, that we must never aim at anything but what is virtuous. The wise man does nothing but what can be done openly and without falseness, nor does he do anything whereby he may involve himself in any wrong-doing, even where he may escape notice. For he is guilty in his own eyes, before being so in the eyes of others; and the publicity of his crime does not bring him more shame than his own consciousness of it. This we can show, not by the made-up stories which philosophers use, but from the true examples of good men.
30. I need not, therefore, imagine a great chasm in the earth, which had been loosened by heavy rains, and had afterwards burst asunder, as Plato does. For he makes Gyges descend into that chasm, and to meet there that iron horse of the fable that had doors in its sides. When these doors were opened, he found a gold ring on the finger of a dead man, whose corpse lay there lifeless. He desiring the gold took away the ring. But when he returned to the king's shepherds, to whose number he belonged, by chance having turned the stone inwards towards the palms of his hands, he saw all, yet was seen by none. Then when he turned the ring to its proper position, he was again seen by all. On becoming conscious of this strange power, by the use of the ring he committed adultery with the queen, killed the king, and took possession of the kingdom after slaying all the rest, who he thought should be put to death, so that they might be no hindrance to him.
31. Give, says Plato, this ring to a wise man, that when he commits a fault he may by its help remain unnoticed; yet he will be none the more free from the stain of sin than if he could not be hid. The hiding-place of the wise lies not in the hope of impunity but in his own innocency. Lastly, the law is not laid down for the just but for the unjust. For the just has within himself the law of his mind, and a rule of equity and justice. Thus he is not recalled from sin by fear of punishment, but by the rule of a virtuous life.
32. Therefore, to return to our subject, I will now bring forward, not false examples for true, but true examples in place of false. For why need I imagine a chasm in the earth, and an iron horse and a gold ring found on the fingers of a dead man; and say that such was the power of this ring, that he who wore it could appear at his own will, but if he did not wish to be seen, he could remove himself out of the sight of those who stood by, so as to seem to be away. This story, of course, is meant to answer the question whether a wise man, on getting the opportunity of using that ring so as to be able to hide his crimes, and to obtain a kingdom,— whether, I say, a wise man would be unwilling to sin and would consider the stain of sin far worse than the pains of punishment, or whether he would use it for doing wickedness in the hope of not being found out? Why, I say, should I need the pretence of a ring, when I can show from what has been done that a wise man, on seeing he would not only be undetected in his sin, but would also gain a kingdom if he gave way to it, and who, on the other hand, noted danger to his own safety if he did not commit the crime, yet chose to risk his own safety so as to be free from crime, rather than to commit the crime and so gain the kingdom.
33. When David fled from the face of King Saul, because the king was seeking him in the desert with three thousand chosen men to put him to death, he entered the king's camp and found him sleeping. There he not only did him no injury, but actually guarded him from being slain by any who had entered with him. For when Abishai said to him: “The Lord has delivered your enemy into your hand this day, now therefore I will slay him,” he answered: “Destroy him not, for who can stretch forth his hand against the Lord's anointed, and be guiltless?” And he added: “As the Lord lives, unless the Lord shall smite him, or his day shall come to die, or he shall die in battle, and it be laid to me, the Lord forbid that I should stretch out my hand against the Lord's anointed.”
34. Therefore he did not suffer him to be slain, but removed only his spear, which stood by his head, and his cruse of water. Then, while all were sleeping, he left the camp and went across to the top of the hill, and began to reproach the royal attendants, and especially their general Abner, for not keeping faithful watch over their lord and king. Next, he showed them where the king's spear and cruse were which had stood at his head. And when the king called to him, he restored the spear, and said: “The Lord render to every man his righteousness and faithfulness, for the Lord delivered you into my hand, but I would not avenge myself on the Lord's anointed.” Even while he said this, he feared his plots and fled, changing his place in exile. However, he never put safety before innocency, seeing that when a second opportunity was given him of killing the king, he would not use the chance that came to him, and which put in his reach certain safety instead of fear, and a kingdom instead of exile.
35. Where was the use of the ring in John's case, who would not have been put to death by Herod if he had kept silence? He could have kept silence before him so as to be both seen and yet not killed. But because he not only could not endure to sin himself to protect his own safety, but could not bear and endure even another's sin, he brought about the cause of his own death. Certainly none can deny that he might have kept silence, who in the case of Gyges deny that he could have remained invisible by the help of the ring.
36. But although that fable has not the force of truth, yet it has this much to go upon, that if an upright man could hide himself, yet he would avoid sin just as though he could not conceal himself; and that he would not hide his person by putting on a ring, but his life by putting on Christ. As the Apostle says: “Our life is hid with Christ in God.” Let, then, no one here strive to shine, let none show pride, let none boast. Christ willed not to be known here, He would not that His Name should be preached in the Gospel while He lived on earth. He came to lie hidden from this world. Let us therefore likewise hide our life after the example of Christ, let us shun boastfulness, let us not desire to be made known. It is better to live here in humility, and there in glory. “When Christ,” it says, “shall appear, then shall we also appear with Him in glory.”
Source: On the Duties of the Clergy (New Advent)