He pointed to the existence of the so-called "Robber Council," the Second Council of Ephesus, to show that this is what the Church (both East and West) historically understood. The Eastern Orthodox, along with many traditional Protestants, accept the so-called Seven Ecumenical Councils:
- First Council of Nicaea, 325 A.D.
- First Council of Constantinople, 381 A.D.
- Council of Ephesus, 431 A.D.
- Council of Chalcedon, 451 A.D.
- Second Council of Constantinople, 553 A.D.
- Third Council of Constantinople, 680-81 A.D.
- Second Council of Nicaea, 787 A.D.
But in between # 3 and # 4, in 449 A.D., was the Second Council of Ephesus. It's remarkable, in that it attempted to declare the Monophysite heresy to be the truth, on behalf of the Church. In the original Council of Ephesus, a Monophysite priest named Eutyches was condemned for refusing to acknowledge the dual natures of Christ. But in the Second Council of Ephesus, the Robber Council declared that Eutyches was the orthodox one, condemning his opponents, and declaring that Christ only has one nature. Now, if both the Council of Ephesus and the Second Council of Ephesus are valid Councils, we'd have a serious problem: the Church would have just proclaimed heresy, contradicting both Herself and Scripture.
But that's not the case: we know the Second Council of Ephesus is invalid, and have known it from the start. As the council was closing, the papal legate (the pope's representative to the Council), Hilarius, expressed the judgment of Rome: "Contradicitur!" With a single word, he declared the Council invalid in the name of the pope. Leo himself confirmed this, and it's from him that we have the name "Robber Council."
There are a few things remarkable about this:
First, it's only on the basis of the papacy that we can reject the Second Council as an invalid Robber Council. That is, it's on this basis alone that we can coherently say why this isn't a Council at all. After all, the Robber Council was attended by about 130 Bishops, just a little less than the First Council of Constantinople, which is considered an Ecumenical Council. And the Robber Council was drawn from a wider swath of Christendom than First Constantinople. So by all appearances, it was an Ecumenical Council. And you can't say it wasn't an Ecumenical Council just because it was heretical, or later condemned. That's circular logic -- a Council isn't a Council if you happen to think it's right. So the only reason we can say that the Robber Council wasn't a true Council, rather than a true Council that decreed error, is by recognizing that the Council.
Second, the Eastern Orthodox accepted the Pope's authority in declaring the Second Council of Ephesus to be invalid. They also deny the Second Council of Ephesus, and refer to it in Pope Leo's terms: as the Robber Council. Historically, the reason the Eastern Orthodox rejected it as a Council wasn't that it was wrong, but because it had been condemned by the papal legate, and then the pope.
Third, those condemned by the Council looked to the Pope to find out the Council's Validity. Bishop Theodoret of Cyprus, one of the men condemned by the Robber Council, appealed to the Pope, and said, "I await your sentence, and if you command me to abide by my condemnation, I will abide by it." This is an Eastern Bishop acknowledging that the Pope, and not an unapproved Council, has the final say.
So in the end, concilarism is certainly false. If Councils don't need papal approval to be valid, then we have to recognize and accept all the Councils. And that's impossible, since the Robber Council explicitly contradicts the Council of Ephesus, and it was explicitly condemned by the Council of Chalcedon. You'd have to declare the Holy Spirit contradictory to cling to this notion.
Instead, the truth is that the pope has the ability to declare which Councils are and are not valid, and led by the Holy Spirit, just as he can declares which Books are and are not Scripture. To simply declare that anything purporting to be a Council is one is as absurd as believing anything which purports to be Scripture.
Post-Script: The Councils of Chalcedon and Quinisext
Two subsequent Councils make these points even clearer. First, the Council of Chalcedon was convened precisely to establish that Christ has two natures. This Council is recognized by the Catholic Church, the Eastern Orthodox, and many Protestants; and all orthodox Christians would say its Christology is correct. After reading into the record St. Leo's "tome," his defense of the dual natures of Christ, the bishops cried out, "Peter has spoken thus through Leo. So taught the Apostles." It's an affirmation of the Petrine ministry, and the headship of the pope, and it was recorded in the proceedings of the Council. And Pope Hadrian (Leo's successor, who had served as the papal legate at the Robber Council) sent a letter discussing the authority of the pope over the Church, which the Council accepted. The Patriarch of Constantinople, Tarasius, then declares the pope's letter to be true and accurate. So in the Ecumenical Council refuting the Robber Council, accepted by Catholics, Orthodox, and Protestants alike, the papacy is established quite plainly.
The second Council worth mentioning is another false Council, the Quinisext Council. It was held in the East, and accepted by the Eastern Orthodox, but rejected by the papacy. The East is careful to recognize it, not as an additional Council, but as additional canons to be added to existing Councils (this way, they can say that they affirm just the first Seven Ecumenical Councils). Two things make it remarkable. First, Basil of Gortyna presented himself as the "papal legate," although he was not. The fact that it was viewed as necessary that there be a papal legate speaks volumes. Second, the Byzantine Emperor, Justinian II, was outraged that the pope refused to accept the council, and actually sent an officer to Rome to kidnap him. Italian troops came down from Ravenna (then the capital of the Western Roman Empire) and stopped him. That the pope, and he alone, was viewed as a significant enough figure to send troops to kidnap him a thousand miles away is telling. If papal approval of the Council was unnecessary, why bother trying to arrest him?