Lying is the intentional speaking against what one thinks, in order to deceive. By its very nature, this is always wrong, because speech is given to reveal thoughts. (STh II-II q.110 a.1)
CCC 2482
“A lie consists in speaking a falsehood with the intention of deceiving.”
CCC 2483
Lying is the gravest violation of the truth. It is the direct opposite of the truth, which one is called to profess.
CCC 2484
The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the liar, and the harm suffered by those affected. If a lie in itself is only a venial sin, it becomes mortal when it constitutes a serious offense against the virtue of justice.
CCC 2485
Lying is by its nature reprehensible. It is a profanation of speech, whose purpose is to communicate known truth. The deliberate falsehood spoken with the intention to mislead is an offense against justice and charity.
CCC 2486
“By a lie, one commits perjury. It causes a split between speech and reality. It is a disturbance of the relationship of man to his neighbor. It violates the fundamental right of people to the truth.”
CCC 2487
“Every offense against the truth requires reparation.”
CCC 2465
“Sacred Scripture attests: ‘God is truth’ (cf. Rom 3:4). […] His people are called to live in the truth that comes from God.”
CCC 2466
“In the Son of God, God’s truth has been made manifest in person. ‘I am the way, and the truth, and the life’ (Jn 14:6). Whoever believes in him lives by the truth.”
CCC 2467
“Man is by nature directed toward the truth. He is obliged to adhere to it and to order his life according to it.”