36 For, concerning false witness, which is set down in the ten commands of the Law, it can indeed in no way be contended that love of truth may at heart be preserved, and false witness brought forth to him unto whom the witness is borne. For, when it is said to God only, then it is only in the heart that the truth is to be embraced: but when it is said to man, then must we with the mouth also of the body bring forth truth, because man is not an inspector of the heart.
But then, touching the witness itself, it is not unreasonably asked, to whom one is a witness? For not to whomsoever we speak unto are we witnesses, but to them to whom it is expedient and due that they by our means should come to know or believe the truth; as is a judge, that he may not err in judging; or he who is taught in doctrine of religion, that he may not err in faith, or by very authority of the teacher waver in doubt. But when the person who interrogates you or wishes to know anything from you seeks that which concerns him not, or which is not expedient for him to know, he craves not a witness, but a betrayer.
Therefore if to him you tell a lie, from false witness perhaps you will be clear, but from a lie assuredly not. So then with this salvo, that to bear false witness is never lawful, the question is, whether it be lawful sometimes to tell a lie. Or if it be false witness to lie at all, it is to be seen whether it admit of compensation, to wit, that it be said for the sake of avoiding a greater sin: as that which is written, “Honor father and mother,” under stress of a preferable duty is disregarded; whence the paying of the last honors of sepulture to a father, is forbidden to that man who by the Lord Himself is called to preach the kingdom of God.
Source: On Lying (New Advent)