Catechism of the Council of Trent: No False Witness
Excuses for Lying Refuted
The Plea Of Prudence
There are those who seek to justify their duplicity either by the unimportance of what they say, or by the example of the worldly wise who, they claim, lie at the proper time. The pastor should correct such erroneous ideas by answering what is most true, namely, that the wisdom of the flesh is death. He should exhort his listeners in all their difficulties and dangers to trust in God, not in the artifice of lying; for those who have recourse to subterfuge, plainly show that they trust more to their own prudence than to the providence of God.
The Plea Of Revenge
Those who lay the blame of their own falsehood on others, who first deceived them by lies, are to be taught the unlawfulness of avenging their own wrongs, and that evil is not to be rendered for evil, but rather that evil is to be overcome by good. Even if it were lawful to return evil for evil, it would not be to our interest to harm ourselves in order to get revenge. The man who seeks revenge by uttering falsehood inflicts very serious injury on himself.
The Pleas Of Frailty, Habit, And Bad Example
Those who plead human frailty are to be taught that it is a duty of religion to implore the divine assistance, and not to yield to human infirmity.
Those who excuse themselves by habit are to be admonished to endeavour to acquire the contrary habit of speaking the truth; particularly as those who sin habitually are more guilty than others.
There are some who adduce in their own justification the example of others, who, they contend, constantly indulge in falsehood and perjury. Such persons should be undeceived by reminding them that bad men are not to be imitated, but reproved and corrected; and that, when we ourselves are addicted to the same vice, our admonitions have less influence in reprehending and correcting it in others.
The Pleas Of Convenience, Amusement, And Advantage
With regard to those who defend their conduct by saying that to speak the truth is often attended with inconvenience, priests should answer that (such an excuse) is an accusation, not a defence, since it is the duty of a Christian to suffer any inconvenience rather than utter a falsehood.
There remain two other classes of persons who seek to justify lying: those who say that they tell lies for the sake of amusement, and those who plead motives of interest, claiming that without recourse to lies, they can neither buy nor sell to advantage. The pastor should endeavour to reform both these kinds of liars. He should correct the former by showing how strong a habit of sinning is contracted by their practice, and by strongly impressing upon them the truth that for every idle word they shall render an account. As for the second class, he should upbraid them with greater severity, because their very excuse is a most serious accusation against themselves, since they show thereby that they yield no faith or confidence to these words of God: Seek first the kingdom of God and his justice, and all these things shall be added unto you.