The Enchiridion

In some things, then, it is a great evil to be deceived; in some it is a small evil; in some no evil at all; and in some it is an actual advantage. It is to his grievous injury that a man is deceived when he does not believe what leads to eternal life, or believes what leads to eternal death. It is a small evil for a man to be deceived, when, by taking falsehood for truth, he brings upon himself temporal annoyances; for the patience of the believer will turn even these to a good use, as when, for example, taking a bad man for a good, he receives injury from him.

But one who believes a bad man to be good, and yet suffers no injury, is nothing the worse for being deceived, nor does he fall under the prophetic denunciation: “Woe to those who call evil good!” For we are to understand that this is spoken not about evil men, but about the things that make men evil. Hence the man who calls adultery good, falls justly under that prophetic denunciation. But the man who calls the adulterer good, thinking him to be chaste, and not knowing him to be an adulterer, falls into no error in regard to the nature of good and evil, but only makes a mistake as to the secrets of human conduct.

He calls the man good on the ground of believing him to be what is undoubtedly good; he calls the adulterer evil, and the pure man good; and he calls this man good, not knowing him to be an adulterer, but believing him to be pure. Further, if by making a mistake one escape death, as I have said above once happened to me, one even derives some advantage from one's mistake. But when I assert that in certain cases a man may be deceived without any injury to himself, or even with some advantage to himself, I do not mean that the mistake in itself is no evil, or is in any sense a good; I refer only to the evil that is avoided, or the advantage that is gained, through making the mistake.

For the mistake, considered in itself, is an evil: a great evil if it concern a great matter, a small evil if it concern a small matter, but yet always an evil. For who that is of sound mind can deny that it is an evil to receive what is false as if it were true, and to reject what is true as if it were false, or to hold what is uncertain as certain, and what is certain as uncertain? But it is one thing to think a man good when he is really bad, which is a mistake; it is another thing to suffer no ulterior injury in consequence of the mistake, supposing that the bad man whom we think good inflicts no damage upon us.

In the same way, it is one thing to think that we are on the right road when we are not; it is another thing when this mistake of ours, which is an evil, leads to some good, such as saving us from an ambush of wicked men.