20 But haply some may think that there is an exception to be added; that there be some honest lies which not only hurt no man, but profit some man, excepting those by which crimes are screened and defended: so that the reason why the aforesaid lie is disgraceful, is that, although it hurt no man, and profit the poor, it screens a theft; but if it should in such sort hurt nobody and profit somebody as not to screen and defend any sin, it would not be morally wrong. As, put the case that some one should in your sight hide his money that he might not lose it by theft or violence, and thereupon being questioned you should tell a lie; you would hurt no man, and would serve him who had need that his money were hidden, and would not have covered a sin by telling a lie.
For it is no sin if a man hide his property which he fears to lose. But, if we therefore sin not in telling a lie, for that, while covering no man's sin, we hurt nobody and do good to somebody, what are we about as concerning the sin itself of a lie? For where it is laid down, “You shall not steal,” there is also this, “You shall not bear false witness.” Since then each is severally prohibited, why is false witness culpable if it cover a theft or any other sin, but if without any screening of sin it be done by itself, then not culpable, whereas stealing is culpable in and by itself, and so other sins? Or is it so that to hide a sin is not lawful; to do it, lawful?
Source: On Lying (New Advent)