The Magisterium teaches infallibly in any of three ways:
1. Papal Infallibility
2. Conciliar Infallibility
3. ordinary and universal Magisterium
These infallible teachings are preserved free from all stain of error by the work of the Holy Spirit. The faithful are required to give each and every infallible teaching of the Magisterium the full assent of faith because each of these teachings is entirely true, and its truth is guaranteed by God.
But from the point of view of the SSPX and various sedevacantist splinter groups, if they judge that an idea is false, and:
1. a Pope exercises Papal Infallibility to teach that the idea is true, they will say that he is not teaching infallibly because he is a heretic;
2. a Council exercises Conciliar Infallibility to teach that the idea is true, they will say that the Council is not valid because the Pope and the body of Bishops at the Council fell into heresy;
3. successive Popes and the body of Bishops in communion with them all teach one position on a matter of faith or morals, definitively to be held, thereby making the teaching infallible under the ordinary and universal Magisterium, they will say that all of these Popes and all of these Bishops are heretics.
In this way, they reject any idea that they judge to be false, based on their own understanding or misunderstanding of past teachings, as if they were each and all above the Magisterium, even to the extent of judging and condemning one Pope after another, and all the thousands of Bishops in communion with each Pope, and all the faithful in communion with them.
The indefectibility of the Church and the infallibility of the Magisterium are gifts from Christ to His people, so that they will not be led astray. But the SSPX (and others like them) have acted and spoken as if the Church were not indefectible and as if the Magisterium were not infallible. For no matter how many Popes in succession teach one and the same doctrine, and no matter how many Bishops in communion with these Popes teach the same, the SSPX continues to claim that their group is the only one that cannot possibly fall into error. It is as if the SSPX is claiming to possess Papal Infallibility, and Conciliar Infallibility, and the infallibility of the ordinary and universal Magisterium, and all this for themselves alone.
Such wicked arrogance will not go unpunished by God.
“Therefore, by divine and Catholic faith all those things are to be believed which are contained in the word of God as found in Scripture and tradition, and which are proposed by the Church as matters to be believed as divinely revealed, whether by her solemn judgment or in her ordinary and universal Magisterium.” — First Vatican Council