41 Either then we are to eschew lies by right doing, or to confess them by repenting: but not, while they unhappily abound in our living, to make them more by teaching also. But let him who thinks this, choose out whereby he may help his fellow man being in peril, to what safety he will, what kinds soever of lies; provided yet even of such men we obtain our demand, that upon no cause must we be carried on to false-swearing and to blaspheming. These wickednesses at least let us judge either greater than deeds of lewdness, or certainly not smaller.
For indeed it is worth thinking of, that very often men, where they suspect them of adultery, challenge their wives to an oath: which surely they would not do, unless they believed that even they who were not afraid to perpetrate adultery, might be afraid of perjury. Because in fact also some lewd women who were not afraid by unlawful embraces to deceive their husbands, have been afraid to call God deceitfully to witness unto those same husbands whom they had deceived. What cause then can there be, that a chaste and religious person should be unwilling by adultery to help a man to baptism, yet be willing to help him by perjury, which even adulterers are wont to dread?
And then, if it be shocking to do this by perjuring one's self, how much rather by blaspheming? Far be it then from a Christian to deny and blaspheme Christ, that he may make another man a Christian; and by losing himself seek to find one, whom, if he teach him such things, he may cause to be lost when found. The book then which is called “the Pound,” you must in this method refute and destroy; namely, that head of it in which they dogmatize that for the purpose of concealing religion a lie may be told, this you shall understand must be the first to be amputated; in such manner, that their testimonies by which they labor to advance the Holy Books as patrons of their lies, you must demonstrate partly not to be lies, partly, even those which are such, to be not meet to be imitated: and if infirmity usurps to herself thus much, that somewhat shall be venially permitted unto her which truth approve not, yet that you unshakenly hold and defend, that in divine religion it is at no time whatever right to tell a lie.
And, as for concealed heretics, that, as we are not to find out concealed adulterers by committing of adulteries, nor murderers by committing of murders, nor practisers of black arts by practising of black arts, so neither must we seek to find out liars by telling lies or blasphemers by blaspheming: according to the reasonings which we have in this volume so copiously set forth, that unto the goal of the same, which we fixed to be in this place, we have with difficulty come at last.
Source: To Consentius: Against Lying (New Advent)