37 Likewise, touching that which is written, “A son which receives the word shall be far from destruction: but receiving, he receives it for himself, and no falsehood proceeds out of his mouth:” some one may say, that what is here set down, “A son which receives the word,” is to be taken for no other than the word of God, which is truth. Therefore, “A son receiving the truth shall be far from destruction,” refers to that which is written, “You will destroy all that speak leasing.”
But when it follows, “Receiving he receives for himself,” what other does this insinuate than what the Apostle says, “But let every man prove his own work, and then he shall have glorying in himself and not in another?” For he that receives the word, that is, truth, not for himself, but for men-pleasing, keeps it not when he sees they can be pleased by a lie. But whoso receives it for himself, no falsehood proceeds out of his mouth: because even when the way to please men is to lie, that man lies not, who receiving the truth not thereby to please them but to please God, has received it for himself.
Therefore there is no reason why it should be said here He will destroy all who speak leasing, but not all leasing: because all lies, universally, are cut off in this saying, “And no falsehood proceeds out of his mouth.” But another says, it is to be so taken as the Apostle Paul took our Lord's saying, “But I say unto you, Swear not at all.” For here also all swearing is cut off; but from the mouth of the heart, that it should never be done with approbation of the will, but through necessity of the weakness of another; that is, “from the evil” of another, when it shows that he cannot otherwise be got to believe what is said, unless faith be wrought by an oath; or, from that “evil” of our own, that while as yet involved in the skins of this mortality we are not able to show our heart: which thing were we able to do, of swearing there were no need.
Though moreover in this whole sentence, if the saying, “A son receiving the word shall be far from destruction,” be said of none other than that Truth, by Whom all things were made, which remains ever incommutable; then, because the doctrine of Religion strives to bring men to the contemplation of this Truth, it may seem that the saying, “And no falsehood proceeds out of his mouth,” is said to this purpose, that he speaks no falsehood that pertains to doctrine. Which sort of lie is upon no compensation whatever to be gone into, and is utterly and before all to be eschewed.
Or if the saying, “No falsehood,” is absurdly taken if it be not referred to every lie, the saying, “From his mouth,” should, as was argued above, be taken to mean the mouth of the heart, in the opinion of him who accounts that sometimes one may tell a lie.
Source: On Lying (New Advent)