CCC 2484
Life in Christ›SECTION TWO THE TEN COMMANDMENTS›CHAPTER TWO YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF›III. Offenses Against Truth
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.