In a broad sense perjury is an unlawful oath, one lacking a condition required for a licit oath, viz, truth, prudence, or justice. In a strict sense, perjury is a false statement supported by an oath. Usually perjury refers only to declaratory oaths, but in some European law systems the willful violation of a promissory oath is treated as perjury. Theologians in general say that if such an oath was sincere when uttered, a violation of the promise would be sinful—the gravity depending on the promise—but would not be perjury.
A lying, or perjured, oath is always a mortal sin because it involves contempt for God and disrespect for His attributes. The perjurer asks God to be a witness to a lie or supposes that God can be deceived. Perjury is one of the most serious offenses against the virtue of religion; it contains the malice of contempt for God. Hence only an imperfection in the act can excuse a perjurer from grave sin, for example, if he lacked sufficient reflection on, or full consent to, the oath or its falsity.
Imprudent or useless oaths usually are venial sins, similar to profane use of the holy names. But perjury in a strict sense is always a grave sin, as is shown in the condemnation of the contrary opinion: "To call God to witness to a small lie is not a great irreverence because of which God would wish to or could condemn a man" (H. Denzinger, Enchiridion symbolorum, ed. A. Schönmetzer [32d ed. Freiburg 1963] 2124). Perjury in a wide sense, where required prudence is lacking, would not normally be a grave sin if it does not violate justice directly, because no serious irreverence is shown to God thereby. But such an oath could be gravely sinful because of scandal. If the virtue of justice is violated, perjury in the broad sense is considered a mortal sin ex genere suo, i.e., one that admits of light matter.
It is not lawful for a private person to seek or receive an oath from one who he is sure will commit perjury. With sufficient reason, however, one may seek and receive an oath without knowing whether the person swearing will do so truly or not.
Formal cooperation in an act of perjury is never lawful because it would make the cooperator share the guilt. Hence he who by command, counsel, promise, etc., induces another to swear falsely is guilty as the principal or as an accessory to the crime. Material cooperation is permissible if there is sufficient reason for it. For example, a public official may demand an oath required by law from one who he knows will swear falsely. In such a case, the public good demands that the oath be administered, even though for this person it is an occasion of perjury. The lawgiver, however, should not lightly multiply demands for sworn statements; otherwise the oath can become a mere formality that is thus deprived of probative value, and the temptation to perjury is thereby increased.
Bibliography: thomas aquinas, Summa theologiae, 2a2ae, 89. n. jung, Dictionnaire de théologie catholique, ed. a. vacant et al., 15 v. (Paris 1903–50; Tables générales 1951–) 14.22:1939–55.
"Perjury." New Catholic Encyclopedia. . Encyclopedia.com. (August 17, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/perjury-0
"Perjury." New Catholic Encyclopedia. . Retrieved August 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/perjury-0