21 If this be absurd, what shall we say? Is it so, that there is no “false witness,” but when one tells a lie either to invent a crime against some man, or to hide some man's crime, or in any way to oppress any man in judgment? For a witness seems to be necessary to the judge for cognizance of the cause. But if the Scripture named a “witness” only so far as that goes, the Apostle would not say, “Yea, and we are found false witnesses of God; because we have testified of God that He raised up Christ: whom He raised not up.” For so he shows that it is false witness to tell a lie, yea, in falsely praising a person.
Or perhaps, does the person who lies then utter false witness when he either invents or hides any man's sin, or hurts any man in whatever way? For, if a lie spoken against a man's temporal life is detestable, how much more one against eternal life? As is every lie, if it take place in doctrine of religion. And it is for this reason that the Apostle calls it false witness, if a man tell a lie about Christ, yea, one which may seem to pertain to His praise. Now if it be a lie that neither invents or hides any man's sin, nor is answered to a question of the judge, and hurts no man, and profits some man, are we to say that it is neither false witness, nor a reprehensible lie?
Source: On Lying (New Advent)