“Under the ancient law, the High Priest [of Israel] did not wear the Rational except when he was vested in the pontifical robes and was entering before the Lord. Thus we do not say that the Pope cannot err in his private opinions, as did John XXII; or be altogether a heretic as perhaps Honorius was. Now when he is explicitly a heretic, he falls ipso facto from his dignity and out of the Church, and the Church must either deprive him or, as some say, declare him deprived, of his Apostolic See, and must say as St. Peter did: let another take his bishopric [as was said of Judas Iscariot, Apostle of Jesus Christ]. When he errs in his private opinions he must be instructed, advised, convinced; as happened with John XXII, who was so far from dying obstinate or from determining anything during his life concerning his opinion, that he died whilst he was making the examination which is necessary for determining in a matter of faith, as his successors delcared in the Extravagantes which begins Benedictus Deus. But when he is clothed with the pontifical garments, I mean when he teaches the whole Church as Shepherd, in general matters of faith and morals, then there is nothing but doctrine and truth.
And in fact everything a king says is not a law or an edict, but that only which a king says as king and as a legislator. So everything the Pope says is not canon law or of legal obligation; he must mean to define and to lay down the law for the sheep, and he must keep the due order and form. Thus we say that we must appeal to him not as to a learned man, for in this he is ordinarily surpassed by some others, but as to the general head and pastor of the Church: and as such we must honor, follow, and firmly embrace his doctrine, for then he carries on his breast the Urim and Thummin, doctrine and truth. And again we must not think that in everything and everywhere his judgement is infallible, but then only when he gives judgement on a matter of faith in questions necessary to the whole Church; for in particular cases which depend on human fact he can err, there is no doubt, though it is not for us to control him in these cases save with all reverence, submission, and discretion. Theologians have said, in a word, that he can err in questions of fact, not in questions of right; that he can err extra cathedram, outside the chair of Peter, that is, as a private individual by writings and bad example.
But he cannot err when he is in cathedra, that is, when he intends to make an instruction and decree for the guidance of the whole Church, when he means to confirm his brethren as supreme pastor, and to conduct them into the pastures of the faith. For then it is not so much man who determines, resolves, and defines as it is the blessed Holy Spirit by man, which Spirit, according to the promise made by Our Lord to the Apostles, teaches all truth to the church, and, as the Greek says and the Church seems to understand in a collect of Pentecost, conducts and directs his Church into all truth; But when that Spirit of truth shall come, he will teach you all truth, or, will lead you into all truth…”
(St. Francis De Sales, The Catholic Controversy, Rule Of Faith – Chapter XIV)
Picture from – http://breviloquia.blogspot.com/2012/01/saint-of-day-francis-de-sales-bishop.html