23 But in all our doings, even good men are very greatly embarrassed in the matter of compensative sins; so that these are not esteemed to be sins, if they have such causes for the which they be done, and in the which it may seem to be rather sin, if they be left undone. And chiefly as concerning lies has it come to this pass in the opinion of men that those lies are not accounted sins, nay rather are believed to be rightly done, when one tells a lie for the benefit of him for whom it is expedient to be deceived, or lest a person should hurt others, who seems likely to hurt unless he be got rid of by lies.
In defense of these kinds of lies, very many examples from holy Scripture are accounted to lend their support. It is not, however, the same thing to hide the truth as it is to utter a lie. For although every one who lies wishes to hide what is true, yet not every one who wishes to hide what is true, tells a lie. For in general we hide truths not by telling a lie, but by holding our peace. For the Lord lied not when He said, “I have many things to say unto you, but you cannot bear them now.” He held His peace from true things, not spoke false things; for the hearing of which truths He judged them to be less fit.
But if He had not indicated this same to them, that is, that they were not able to bear the things which He was unwilling to speak, He would indeed hide nevertheless somewhat of truth but that this may be rightly done we should perhaps not know, or not have so great an example to confirm us. Whence, they who assert that it is sometimes meet to lie, do not conveniently mention that Abraham did this concerning Sarah, whom he said to be his sister. For he did not say, She is not my wife, but he said, “She is my sister;” because she was in truth so near akin, that she might without a lie be called a sister.
Which also afterwards he confirmed, after she had been given back by him who had taken her, answering him and saying, “And indeed she is my sister, by father, not by mother;” that is, by the father's kindred, not the mother's. Somewhat therefore of truth he left untold, not told anything of falsehood, when he left wife untold, and told of sister. This also did his son Isaac: for him too we know to have gotten a wife near of kin. It is not then a lie, when by silence a true thing is kept back, but when by speech a false thing is put forward.
Source: To Consentius: Against Lying (New Advent)