Why do you think we are ascertaining beliefs? We just simply accept what was taught by the Church. We don't try to figure it out on our own. If you mean that we have to determine whether or not a theologian's writings conform to Church teaching, that was always the case. An imprimater and nihil obstat doesn't make a book infallible. It just means that a Church official has read the book and determined (not infallibly) that it doesn't contain any error. And that is usually good enough. But theologians have always argued back and forth on various issues. But no one can deny any dogma nor assert that there is error in Sacred Scripture. So we are not ascertaining what we ought to believe. We just accept whatever the Church has taught us.
Ah, but therein lies the very crux of the issue. Perhaps a historical example or two ought to demonstrate the paradox we are in.
Vatican I was rejected by the Old Catholics because they thought that Papal infallibility contradicted what the Church always taught about the nature of the Papacy. Likewise and in the same spirit, Savonarola declared Alexander VI a heretical imposter who may never had the Papacy to begin with because his Renaissance magisterial spirit was deemed to have contradicted prior Church beliefs and teachings and he likewise declared many of those in communion with him as heretics. There are many other examples which may be provided, some of which Cajetan wrote about in his ecclesiological writings when discussing UPA.
The point here is that, ultimately, we are also determining what the Church has taught, its interpretation and application thereof while instrumentalizing that knowledge in our judgements of our current situation.
This is not a new issue. Pope Leo XIII succinctly stated it when he said: “It is also a proof of insincere submission to establish an opposition between one Supreme Pontiff and another Supreme Pontiff. Those who, between two different directions, reject the present one and stick to the past do not show obedience to the authority, which has the right and duty to direct them. In some respects, they resemble those who, after a condemnation, would like to appeal to a future council or a better informed pope.”
In terms of rejecting the very authority which imposes a doctrine which is believed to be in contradiction with what the Church previously believed, Pius IX declared of the Old Catholics:
“Incredibly, they boldly affirm that the Roman Pontiff and all the bishops, the priests and the people conjoined with him in the unity of faith and communion fell into heresy when they approved and professed the definitions of the Ecuмenical Vatican Council. Therefore they deny also the indefectibility of the Church and blasphemously declare that it has perished throughout the world and that its visible Head and the bishops have erred. They assert the necessity of restoring a legitimate episcopacy in the person of their pseudo-bishop, who has entered not by the gate but from elsewhere like a thief or robber and calls the damnation of Christ upon his head.”
Just switch Vatican I with Vatican II, Pius IX with Paul VI, Lefebvre and Thuc with the Old Catholic bishop, Old Catholics with Trads, and voila, you have the Traditionalist movement condemned under the same principles and concepts. Regardless of whether you take the position that the election was null and void in the first place with Paul VI and John XXIII. Some Old Catholics state that Pius IX was a Freemason whose election was never valid to begin with.
It’s a massive issue.