ST. THOMAS ON JUDGMENT BY USURPATION
For those to whom it is not evident that a public judgment must come from the public authorities who have the competency to render the judgment, we can turn to St. Thomas for help. Regarding this point, he wrote:
“Since judgment should be pronounced according to the written law, as stated above, he that pronounces judgment, interprets, in a way, the letter of the law, by applying it to some particular case. Now since it belongs to the same authority to interpret and to make a law, just as a law cannot be made except by public authority, so neither can a judgment be pronounced except by public authority, which extends over those who are subject to the community.”
Only the public authorities have the right to pronounce a public judgment that affects those of the society. St. Thomas goes on to explain that those who render a judgment they have no authority to make are guilty of the unlawful act known as "judgment by usurpation".
“Judgment is lawful in so far as it is an act of justice. Now it follows from what has been stated above (1, ad 1,3) that three conditions are requisite for a judgment to be an act of justice: first, that it proceed from the inclination of justice; secondly, that it come from one who is in authority; thirdly, that it be pronounced according to the right ruling of prudence. If any one of these be lacking, the judgment will be faulty and unlawful. First, when it is contrary to the rectitude of justice, and then it is called ‘perverted’ or ‘unjust’: secondly, when a man judges about matters wherein he has no authority, and this is called judgment ‘by usurpation’; thirdly, when the reason lacks certainty, as when a man, without any solid motive, forms a judgment on some doubtful or hidden matter, and then it is called judgment by ‘suspicion’ or ‘rash’ judgment.”
We see that rendering a public judgment one has no authority to make is an act of injustice. St. Thomas further explains that acts of injustice are, of their genus, a mortal sin.
“I answer that, As stated above (I-II:12:5), when we were treating of the distinction of sins, a mortal sin is one that is contrary to charity which gives life to the soul. Now every injury inflicted on another person is of itself contrary to charity, which moves us to will the good of another. And so since injustice always consists in an injury inflicted on another person, it is evident that to do an injustice is a mortal sin according to its genus.
In light of this, we must conclude that if an individual were to render the public judgment (publicly declare) that a Pope had fallen into heresy and thereby lost his office, it would constitute a mortal sin.