5 But whether a lie be at some times useful, is a much greater and more concerning question. Whether, as above, it be a lie, when a person has no will to deceive, or even makes it his business that the person to whom he says a thing shall not be deceived although he did wish the thing itself which he uttered to be false, but this on purpose that he might cause a truth to be believed; whether, again, it be a lie when a person willingly utters even a truth for the purpose of deceiving; this may be doubted.
But none doubts that it is a lie when a person willingly utters a falsehood for the purpose of deceiving: wherefore a false utterance put forth with will to deceive is manifestly a lie. But whether this alone be a lie, is another question. Meanwhile, taking this kind of lie, in which all agree, let us inquire, whether it be sometimes useful to utter a falsehood with will to deceive. They who think it is, advance testimonies to their opinion, by alleging the case of Sarah, who, when she had laughed, denied to the Angels that she laughed: of Jacob questioned by his father, and answering that he was the elder son Esau: likewise that of the Egyptian midwives, who to save the Hebrew infants from being slain at their birth, told a lie, and that with God's approbation and reward: and many such like instances they pick out, of lies told by persons whom you would not dare to blame, and so must own that it may sometimes be not only not blameworthy, but even praiseworthy to tell a lie.
They add also a case with which to urge not only those who are devoted to the Divine Books, but all men and common sense, saying, Suppose a man should take refuge with you, who by your lie might be saved from death, would you not tell it? If a sick man should ask a question which it is not expedient that he should know, and might be more grievously afflicted even by your returning him no answer, will you venture either to tell the truth to the destruction of the man's life, or rather to hold your peace, than by a virtuous and merciful lie to be serviceable to his weak health? By these and such like arguments they think they most plentifully prove, that if occasion of doing good require, we may sometimes tell a lie.
Source: On Lying (New Advent)