Msgr. Ronald A Knox explains the rational of the Resistance in a sermon from 1931. Speaking of ecuмenical activities then occurring amongst Protestants and the schismatic eastern churches:

       “The Catholic Church stands aloof as she has always stood aloof from every movement which threatens to sacrifice the unity of Christian truth for the sake of peace and good fellowship. If the Anglicans were restored to full communion with the Orthodox Christians of the east tomorrow, and if through that alliance, their ministry recovered the validity of the orders which it lost at the Reformation, they would still be no nearer to reunion with the Catholic Church. Now, why is that? What is it that dictates this exclusive attitude- so our critics call it- to the authorities of the Catholic Church?
   Many non-Catholics believe, others have persuaded themselves to believe, that such reunion will come in the course of time. “You tell us it is impossible”, they say, “but impossibilities, historical impossibilities, are always happening, as you told us just now. And the reason, (they say) why you Catholics are so unsympathetic in England at present is because, at present, you are winning. ..And because for the moment you are winning, you can afford to despise our overtures…
   But you know, it isn’t that. As God sees us, it isn’t that. If you or I were the last Catholic left in England, it would still be impossible for you or me to make terms with Anglicanism, or with any other Christian body. If we did not believe that, we would open negotiations tomorrow. But for us there is a fatal obstacle. All this talk about reunion assumes, necessarily, that the One Church which Christ founded on earth has, in process of time, been split into a set of fragments – the Easterns, the Anglicans, the Wesleyans and so on. Each of these will have preserved some part of Christian truth, some more than others. But since it is impossible now to determine which was right in the old dead controversies, we ought  to sink our differences as far as possible (it is argued) and remake the Church out of its fragments once more. To us Catholics such language is meaningless; we have not so learned Christ. For us, the Church is nothing less than His Temple, His bride, His own Body; and the idea that it could, by any conceivable historical circuмstances, be split into fragments is a blasphemy. The reunion of the Churches is to us not merely impossible, it is unthinkable. You cannot reunite what has never been divided. For the Catholic Church to take part in a reunion of the Churches would be a paradox, a contradiction in terms.
   For that reason, and not for any other, we stand outside the reunion movement…”
 
Msgr. Knox’s testimony is exceptionally powerful. How so? He was not a pope, not a Doctor of the Church, he is not yet a canonized Saint, what is the reason his testimony carries such enormous weight? Because it is not his testimony, per se. Msgr. Knox merely repeated what every pope, every Doctor of the Church, every saint had believed and taught before him. He merely spoke the perennial teaching of the Church. And so Msgr. Knox’s teaching is precisely Archbishop Lefebvre’s teaching. Again not the Archbishop’s per se, as his tomb testifies, the Archbishop only handed on what he had received. And so the Society had taught until Bishop Fellay and his inner circle got the idea from certain European Jєωs (along with large financial bequests) that he and the Society were somehow separated from the Church by holding fast to what had always been believed. That the Society needed to “dialogue” with new Rome, to lay aside differences and restore a unity that the Church had somehow lost.
   But no member of the Society, no member of the Catholic Church at all, who adheres without change to the whole of the Catholic Faith, as once revealed by Christ to the Apostles, has left the Church. Nor need such a one dialogue or compromise with any heretic or error.  Msgr. Knox makes reference that even if the Anglicans were to recover the validity of their orders through union with the Easterns they would be no nearer to union with the Catholic Church. Just so, let Bishop Fellay recognize the “legitimacy” of the novus ordo, it will still be a “rite batard”, and no closer to the Catholic Mass.

Let Bishop Fellay go to modern Rome to reconcile the errors of religious liberty and ecuмenism with the Catholic Faith. To the Catholics of the Resistance such an idea, as Msgr. Knox said, “is unthinkable”, “we have not so learned Christ”.