11 But if any man supposes that the reason why it is right for a person to tell a lie for another is, that he may live the while, or not be offended in those things which he much loves, to the end he may attain unto eternal truth by being taught: that man does not understand, in the first place, that there is no flagitious thing which he may not upon the same ground be compelled to commit, as has been above demonstrated; and in the next place, that the authority of the doctrine itself is cut off and altogether undone if those whom we essay to bring thereunto, are by our lie made to think that it is somewhiles right to lie.
For seeing the doctrine which brings salvation consists partly in things to be believed, partly in things to be understood; and there is no attaining unto those things which are to be understood, unless first those things are believed, which are to be believed; how can there be any believing one who thinks it is sometimes right to lie, lest haply he lie at the moment when he teaches us to believe? For how can it be known whether he have at that moment some cause, as he thinks, for a well-meant lie, deeming that by a false story a man may be frightened and kept from lust, and in this way account that by telling a lie he is doing good even in spiritual things?
Which kind of lie once admitted and approved, all discipline of faith is subverted altogether; and this being subverted, neither is there any attaining to understanding, for the receiving of which that discipline nurtures the babes: and so all the doctrine of truth is done away, giving place to most licentious falsehood, if a lie, even well-meant, may from any quarter have place opened for it to enter in. For either whoso tells a lie prefers temporal advantages, his own or another's, to truth; than which what can be more perverse?
Or when by aid of a lie he wishes to make a person fit for gaining the truth, he bars the approach to truth, for by wishing when he lies to be accommodating, it comes to pass that when he speaks the truth, he cannot be depended upon. Wherefore, either we must not believe good men, or we must believe those whom we think obliged sometimes to tell a lie, or we must not believe that good men sometimes tell lies: of these three the first is pernicious, the second foolish; it remains therefore that good men should never tell lies.
Source: On Lying (New Advent)