It does sound scary, considering virtually every council has been rejected by some in the church, indeed at times even leading to schism, yet the authoritativeness of the councils did not lie in wait, pending the acceptance of everyone. Indeed, had that been the case, it would be hard to imagine how/why any schism should result from the teaching of an ecumenical council at all. Afterall, wouldn’t the teaching be non-binding and lack any authority until everyone accepted it, even if it took 1,500 years?
But the idea that councils are immediately authoritative doesn’t match too well with history. There is absolutely no set of criteria which guarantees that a council will be authoritative. Some Orthodox have (incorrectly) put forth the idea that an Ecumenical Council must be called by the Emperor and approved by the patriarchs of all of the Churches. Catholics like to say that an Ecumenical Council must be universal amongst the Church and approved by a pope.
Both sets of criteria can easily be discredited by the unusual events of the Councils concerning Patriarch Photius. Both the council of 869 and the council of 879 were called by an emperor and received papal and patriarchal approval, yet the Catholic Church rejects that the council of 879 is Ecumenical (largely because of an accident of history; the Catholic Church preserved the acts of Constantinople 869, which contained a canon which would help end the investiture controversy), while the Orthodox Church rejects the council of 869, saying that the council of 879 abrogated it (accordingly, no acts of the 869 council exist in Greek, as they were destroyed after the council of 879 abrogated the council of 869). Who is right? Clearly the issue of what makes a council authoritative is more complicated than meeting a certain set of criteria.
But should ordinary laity unprotected by Christ’s promises to his church of infallibility be allowed to determine the veracity of a council’s teachings?
Imagine this scenario: (1) such a gathering takes place in the American state of Massachusetts concerning the morality of artificial contraception; (2) the attending Catholic bishops vote in favour of lifting all restrictions on the practice; and (3) the laity of the world–after being surveyed by professional researchers–unsurprisingly turn out to agree in overwhelming numbers with the conclusions just reached. Would average observers then be justified in believing that artificial contraception–under Eastern Orthodox ecclesiology–was entirely acceptable? Or would the inevitable rejection of this council among the African and Asian laity cancel out the opinions of their Western brethren?
No, it’s not like that at all. The Church is not a democracy (not even at the conciliar level), and the laity do not have that sort of authority. I did not mean to say that the laity have the ability to determine the authority of a council. It is a more of a mysterious thing than that. At times, the Church acts with one mind, and such events are seen to be a sign that the Spirit is guiding the Church.
The very notion that a council can be ‘infallible’ is a rather dangerous way to think of it. If we are to believe that councils only exist to clarify the faith of the Fathers, and not to create new teachings, then the notion of infallibility is a moot point, because it is the agreement of the council with the deposit of the faith (represented by the teachings which have been faithfully passed down by the body of the bishops who are successors of the Apostles) which confers any sort of infallibility to a council, not the council meeting some sort of magic criteria for infallibility. This is why we see, for example, the bishops at the Council of Chalcedon reading and agreeing with the Creed of Nicaea, the Creed of Constantinople and the Second Letter of Cyril to Nestorius: the bishops are simultaneously expressing their agreement with the documents, and also affirming (or rather reaffirming) that the documents are in accord with the faith of the Fathers and of the Apostles.
To answer your hypothetical question, we would have to wait and see. If we truly are to believe that the Holy Spirit will protect the Church from error, then we must let those issues play out over time. For the Orthodox, mob rule is just as an unacceptable style of governance as requiring the pope to give infallibility to a council, for sometimes those who were correct were in the minority (think Athanasius contra mundum or Maximus the Confessor), and sometimes those who were in power were in error (Nestorius, Honorius, and Sergius to name a few), but by the working of the Holy Spirit in the Church, the Truth managed to prevail.