Much needs to be said about the Church’s infallibility, especially to correct illusions arising (by mistake) from the Definition of Papal infallibility in 1870. Today for instance sedevacantists and liberals think that their positions are wholly opposed, but do they stop for a moment to see how similarly they think?—Major: Popes are infallible. Minor: Conciliar Popes are liberal. Liberal Conclusion: we must become liberal. Sedevacantist Conclusion: they cannot be Popes. The error is neither in the logic, nor in the Minor. It can only be in a misunderstanding on both their parts of infallibility in the Major. Once again, modern men put authority above truth.
Eternal God is Truth itself, absolutely infallible. In created time, through his Incarnate Son, he instituted his Church with a doctrine for the salvation of human souls. Coming from him that doctrine could only be inerrant, but to keep it free from the errors of the human churchmen to whom he would entrust it, his Son promised them the “spirit of truth” to guide them “for ever” (Jn. XIV, 16). For indeed without some such guarantee, how could God require of men, on pain of eternal damnation, to believe in his Son, in his doctrine and in his Church (Mk.XVI, 16)?
Yet even from churchmen God will not take away that free-will to err which he gave them. And he will allow that freedom to go as far as they wish, short of their making his Truth inaccessible to men. That reaches far, and it includes a number of highly defective Popes, but God’s reach is still farther than the wickedness of men (Isaiah LIX, 1,2). At Vatican II for instance, Church error went a long way, without however God’s allowing his Church to be wholly defectible in its presentation to men of the inerrant Truth coming from his own infallibility. Even the Conciliar Popes have told many Catholic truths alongside their Conciliar errors.
But how then can I, a simple soul, tell the difference between their truths and their errors? Firstly, if I am truly looking for God with an upright heart, he will guide me to him, as the Bible says in many places. And secondly, God’s doctrine being as unchangeable as God, it must be the doctrine that I find (nearly) all his churchmen to have taught and handed down in (nearly) all places and at (nearly) all times, best known as Tradition. From the beginning of the Church, that handing down has been the surest test of what Our Lord himself taught. Down the ages inerrant Tradition has been the work of millions of churchmen. It has been that for which God endowed his Church as a whole, and not just the Popes, with the guidance of the infallible Holy Ghost.
Here is, so to speak, the cake of Church infallibility upon which the Popes’ solemn Definitions are merely the icing, precious and necessary, the peak of the Church’s infallibility, but not its mountain bulk. Notice firstly that Definitions by the Popes’ Extraordinary Magisterium existed not only from 1870 but from the beginning of the Church, and they existed not to make Tradition true but merely to make certain what belonged to Tradition and what did not, whenever the erring of men had made that uncertain. Sensing truth, Archbishop Lefebvre rightly preferred inerrant Tradition to gravely erring Popes. Never having understood him, like all modern liberals not sensing truth, his successors are in the process of preferring erring Popes to inerrant Tradition. Underestimating truth and overestimating the Popes, sedevacantists wholly repudiate the erring Popes and can be tempted to quit the Church altogether. Lord, have mercy!