The Catechism on Lying:
2483 Lying is the most direct offense against the truth. To lie is to speak or act against the truth in order to lead someone into error. By injuring man’s relation to truth and to his neighbor, a lie offends against the fundamental relation of man and of his word to the Lord.
2484 The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity.
2485 By its very nature, lying is to be condemned. It is a profanation of speech, whereas the purpose of speech is to communicate known truth to others. The deliberate intention of leading a neighbor into error by saying things contrary to the truth constitutes a failure in justice and charity. The culpability is greater when the intention of deceiving entails the risk of deadly consequences for those who are led astray.
2486 Since it violates the virtue of truthfulness, a lie does real violence to another. It affects his ability to know, which is a condition of every judgment and decision. It contains the seed of discord and all consequent evils. Lying is destructive of society; it undermines trust among men and tears apart the fabric of social relationships.
Aquinas on Lying
Objection 4. Further, one ought to choose the lesser evil in order to avoid the greater: even so a physician cuts off a limb, lest the whole body perish. Yet less harm is done by raising a false opinion in a person’s mind, than by someone slaying or being slain. Therefore a man may lawfully lie, to save another from committing murder, or another from being killed.
Reply to Objection 4. A lie is sinful not only because it injures one’s neighbor, but also on account of its inordinateness, as stated above in this Article. Now it is not allowed to make use of anything inordinate in order to ward off injury or defects from another: as neither is it lawful to steal in order to give an alms [. . .] Therefore it is not lawful to tell a lie in order to deliver another from any danger whatever. Nevertheless it is lawful to hide the truth prudently, by keeping it back, as Augustine says (Contra Mend. x).
ST II-II, Q. 110, A. 3.