18. Here a most difficult and complex issue arises which I once dealt with in a large book, in response to the urgent question whether it is ever the duty of a righteous man to lie.  Some go so far as to contend that in cases concerning the worship of God or even the nature of God, it is sometimes a good and pious deed to speak falsely. It seems to me, however, that every lie is a sin, albeit there is a great difference depending on the intention and the topic of the lie. He does not sin as much who lies in the attempt to be helpful as the man who lies as a part of a deliberate wickedness. Nor does one who, by lying, sets a traveler on the wrong road do as much harm as one who, by a deceitful lie, perverts the way of a life. Obviously, no one should be adjudged a liar who speaks falsely what he sincerely supposes is the truth, since in his case he does not deceive but rather is deceived. Likewise, a man is not a liar, though he could be charged with rashness, when he incautiously accepts as true what is false. On the other hand, however, that man is a liar in his own conscience who speaks the truth supposing that it is a falsehood. For as far as his soul is concerned, since he did not say what he believed, he did not tell the truth, even though the truth did come out in what he said. Nor is a man to be cleared of the charge of lying whose mouth unknowingly speaks the truth while his conscious intention is to lie. If we do not consider the things spoken of, but only the intentions of the one speaking, he is the better man who unknowingly speaks falsely-because he judges his statement to be true-than the one who unknowingly speaks the truth while in his heart he is attempting to deceive. For the first man does not have one intention in his heart and another in his word, whereas the other, whatever be the facts in his statement, still "has one thought locked in his heart, another ready on his tongue,"  which is the very essence of lying. But when we do consider the things spoken of, it makes a great difference in what respect one is deceived or lies. To be deceived is a lesser evil than to lie, as far as a man's intentions are concerned. But it is far more tolerable that a man should lie about things not connected with religion than for one to be deceived in matters where faith and knowledge are prerequisite to the proper service of God. To illustrate what I mean by examples: If one man lies by saying that a dead man is alive, and another man, being deceived, believes that Christ will die again after some extended future period-would it not be incomparably better to lie in the first case than to be deceived in the second? And would it not be a lesser evil to lead someone into the former error than to be led by someone into the latter?
19. In some things, then, we are deceived in great matters; in others, small. In some of them no harm is done; in others, even good results. It is a great evil for a man to be deceived so as not to believe what would lead him to life eternal, or what would lead to eternal death. But it is a small evil to be deceived by crediting a falsehood as the truth in a matter where one brings on himself some temporal setback which can then be turned to good use by being borne in faithful patience-as for example, when someone judges a man to be good who is actually bad, and consequently has to suffer evil on his account. Or, take the man who believes a bad man to be good, yet suffers no harm at his hand. He is not badly deceived nor would the prophetic condemnation fall on him: "Woe to those who call evil good." For we should understand that this saying refers to the things in which men are evil and not to the men themselves. Hence, he who calls adultery a good thing may be rightly accused by the prophetic word. But if he calls a man good supposing him to be chaste and not knowing that he is an adulterer, such a man is not deceived in his doctrine of good and evil, but only as to the secrets of human conduct. He calls the man good on the basis of what he supposed him to be, and this is undoubtedly a good thing. Moreover, he calls adultery bad and chastity good. But he calls this particular man good in ignorance of the fact that he is an adulterer and not chaste. In similar fashion, if one escapes an injury through an error, as I mentioned before happened to me on that journey, there is even something good that accrues to a man through his mistakes. But when I say that in such a case a man may be deceived without suffering harm therefrom, or even may gain some benefit thereby, I am not saying that error is not a bad thing, nor that it is a positively good thing. I speak only of the evil which did not happen or the good which did happen, through the error, which was not caused by the error itself but which came out of it. Error, in itself and by itself, whether a great error in great matters or a small error in small affairs, is always a bad thing. For who, except in error, denies that it is bad to approve the false as though it were the truth, or to disapprove the truth as though it were falsehood, or to hold what is certain as if it were uncertain, or what is uncertain as if it were certain? It is one thing to judge a man good who is actually bad-this is an error. It is quite another thing not to suffer harm from something evil if the wicked man whom we supposed to be good actually does nothing harmful to us. It is one thing to suppose that this particular road is the right one when it is not. It is quite another thing that, from this error-which is a bad thing-something good actually turns out, such as being saved from the onslaught of wicked men.
 Ad consentium contra mendacium, CSEL (J. Zycha, ed.), Vol. 41, pp. 469-528; also Migne, PL, 40, c. 517-548; English translation by H.B. Jaffee in Deferrari, St. Augustine: Treatises on Various Subjects (The Fathers of the Church, New York, 1952), pp. 113-179. This had been written about a year earlier than the Enchiridion. Augustine had also written another treatise On Lying much earlier, c. 395; see De mendacio in CSEL (J. Zycha, ed.), Vol. 41, pp. 413-466; Migne, PL, 40, c. 487-518; English translation by M.S. Muldowney in Deferrari, op. cit., pp. 47-109. This summary of his position here represents no change of view whatever on this question.