First, again, please keep in mind the three requirements for mortal sin: grave matter, full knowledge, full and free consent of the will (cf. CCC 1857). If all three of those requirements are not met, at most the sin would be venial if it was a sin at all (cf. CCC 1862). Since we cannot assess any individual’s personal knowledge and consent (only the individual himself, perhaps with the assistance of a spiritual director, can do that), the question is, at what point does lying become grave matter?
In the English language the word lie covers a spectrum of non-truths from the harmless (e.g., Santa’s existence) to the malicious. Traditionally speaking, the word lie has not been used in English translations of the Ten Commandments, perhaps because it does not give an exact understanding of what the commandment against lying is forbidding; the traditional phrase is “bearing false witness” (i.e., falsely accusing another person of a misdeed). Quoting Jesus’ words to the rich young man seeking perfection, the Catechism of the Catholic Church defines grave matter as, “bearing false witness” and “fraud”; the more general untruths usually covered by the word lying are not included:
Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man (cf. Luke 18:18-23): “Do not kill, do not commit adultery, do not steal, do not bear false witness, do not defraud, honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger (CCC 1858).
Non-truths that do not offend justice and are usually used to spare another’s feelings or to indulge fantasy and imagination are not lies. Untruths, usually used to protect the individual or another from unwanted consequences, may offend justice if they are told without just cause and therefore may constitute venial sin. Malicious untruths, told to harm another individual, are grave matter and may constitute mortal sin if the other requirements for mortal sin are met.
Can you reserve the truth?
Is it okay to lie under certain circumstances?**
Getting Kids Out of Harm’s Way by Jimmy Akin