5 And how will you do this that I say, “Cut him off, and cast him from you,” and so, it may be, amend him? Answer me, how you are going to do it? He wished to persuade you out of the Law to tell a lie. For he said, “speak.” And perhaps he did not dare to say, “speak a lie;” but thus, “speak what the other wishes.” You say, “But it is a lie.” And he to excuse it, says, “All men are liars.” Then do you, my brother, say against this, “The mouth that lies slays the soul.” Mark, it is no light thing you have heard, “The mouth that lies slays the soul.”
What can that powerful enemy, who oppresses me, do to me, that you pity me, and my condition, and would not have me be in this evil case; whereas you would that I should be evil? What can that powerful man do to me, and what can he oppress? The flesh. He can oppress your body, you will say: I grant he may oppress it to destruction. Still how much more mildly does he deal with me, than I should with myself were I to lie! He kills my flesh; I kill my soul. He in his power and anger slays the body; “the mouth that lies slays the soul.”
He slays the body; and die it must, though it should not be slain; but the soul which iniquity slays not, the truth receives for ever. Preserve then what you can preserve; and let that perish which must perish sometime or other. You have given an answer then, but you have not solved the “All men are liars.” Make answer to him to this too, that he may not fancy that he has said anything to persuade to lying, in bringing a testimony out of the Law; so urging you out of the Law against the Law.
For it is written in the Law, “You shall not bear false witness;” and it is written in the Law, “All men are liars.” Recur then to that which I just lately suggested, when I defined in words as best I could the “meek” man. He is “meek” to whom in all things that he does well, nothing but God is pleasing, and in all the evils which he suffers, God is not displeasing. Make answer then to him who says, Lie, for it is written, “All men are liars:” I will not lie, for it is written, “The mouth that lies slays the soul.”
I will not lie, because it is written, “You shall destroy them that speak lying.” I will not lie, because it is written, “You shall not bear false witness.” Though he whom I displease by the truth harass my body with oppressions, I will give ear to my Lord, “Fear not them which kill the body.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)