39 And all these sins, truly, whether such whereby an injury is done to men in the comforts of this life, or whereby men corrupt themselves and hurt none against his will: all these sins, then, even though they seem to mean well by this temporal life to the procuring of any delight or profit, (for no man commits any of these things with any other purpose and end;) yet in regard of that life which is forever and ever, they do entangle and in all ways hinder. But there are some of these that hinder the doers only, others likewise those on whom they are done.
For as to the things which people keep safe for the sake of utility to this life, when these are taken away by injurious persons, they alone sin and are hindered from eternal life who do this, not they to whom they do it. Therefore, even if a person consent to the taking of them from him, either that he may not do some evil, or that he may not in these very things suffer some greater inconvenience; not only does he not sin, but in the one case he acts bravely and laudably, in the other usefully and unblameably.
But as to those things which are kept for the sake of sanctity and religion, when injurious persons wish to violate these, it is right, if the condition be proposed and the means given, to redeem them even by sins of lesser moment, yet not by wrongs to other men. And then do these things thenceforth cease to be sins, which are undertaken in order to the avoidance of greater sins. For as in things useful, for instance in pecuniary or any other corporal commodity, that is not called a loss which is parted with in order to a greater gain; so in things holy, that is not called sin which is admitted lest a worse be admitted.
Or if that is called loss, which one foregoes that he may not forego more; let this also be called sin, while however the necessity of undertaking it in order to the eschewing of a greater is no more to be doubted, than that, in order to avoid a greater loss, it is right to suffer a smaller one.
Source: On Lying (New Advent)