The fallacy of an “appeal to authority” is one in which a person, to make an opinion *appear* credible, appeals to the opinion of an *apparent* authority. However, the authority appealed to is actually not capable of adding credibility to the opinion.
A perfect example of this fallacy, which we see every day across social media, is found where “Trad” Catholics present their opinions using a bishop or priest as their mouthpiece.
You will find that they always quote from a few, isolated bishops or priests–Vigano, Schneider, etc.–as if quoting an individual member of the clergy *proves* the opinion to be “Church teaching”.
The fallacy in this is that the authority in the Church is with the Magisterium, which is not present with any individual bishop or priest, but with the Pope and bishops in union with the Pope.
The Catechism of the Catholic Church teaches:
“The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ.” This means that the task of interpretation has been entrusted to THE BISHOPS IN COMMUNION WITH THE SUCCESSOR OF PETER, the Bishop of Rome.”
Catechism of the Catholic Church, par. 85
Thus, having an individual, isolated bishop speak against the Pope and other bishops in support of your opinion is a fallacious “appeal to authority” because that isolated bishop or priest does not have the authority to speak authoritatively on behalf of the Church.
An honest man would never make such an appeal. It is the inclination of a dishonest, or “sophistical” man who would attempt to mislead others by such an appeal to another as a false source of credibility for a false opinion that has no merit of its own.