31 But he who says that some lies are just, must be judged to say no other than that some sins are just, and therefore some things are just which are unjust: than which what can be more absurd? For whence is a thing a sin, but for that it is contrary to justice? Be it said then that some sins are great, some small, because it is true; and let us not listen to the Stoics who maintain all to be equal: but to say that some sins are unjust, some just, what else is it than to say that there be some unjust, some just iniquities?
When the Apostle John says, “Every man who does sin, does also iniquity and sin is iniquity.” It is impossible therefore that a sin should be just, unless when we put the name of sin upon another thing in which one does not sin, but either does or suffers anything for sin. Namely, both sacrifices for sins are named “sins,” and the punishments of sins are sometimes called sins. These doubtless can be understood to be just sins, when just sacrifices are spoken of, or just punishments.
But those things which are done against God's law cannot be just. It is said unto God, “Your law is truth:” and consequently, what is against truth cannot be just. Now who can doubt that every lie is against truth? Therefore there can be no just lie. Again, what man does not see clearly that every thing which is just is of the truth? And John cries out, “No lie is of the truth.” No lie therefore is just. Wherefore, when from holy Scriptures are proposed to us examples of lying, either they are not lies, but are thought to be so while they are not understood; or, if lies they be, they are not meet to be imitated, because they cannot be just.
Source: To Consentius: Against Lying (New Advent)