---Cardinal Joseph Ratzinger, CDF, DECLARATION "DOMINUS IESUS" (August 2000): Above all else, it must be firmly believed that “the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is the mediator and the way of salvation; he is present to us in his body which is the Church. He himself explicitly asserted the necessity of faith and baptism (cf. Mk 16:16; Jn 3:5), and thereby affirmed at the same time the necessity of the Church which men enter through baptism as through a door”.
---Saint John Paul II (UT UNUM SINT, MAY 1995): Here it is not a question of altering the deposit of faith, changing the meaning of dogmas, eliminating essential words from them, accommodating truth to the preferences of a particular age, or suppressing certain articles of the Creed under the false pretext that they are no longer understood today. The unity willed by God can be attained only by the adherence of all to the content of revealed faith in its entirety. In matters of faith, compromise is in contradiction with God who is Truth. In the Body of Christ, "the way, and the truth, and the life" (Jn 14:6), who could consider legitimate a reconciliation brought about at the expense of the truth?
---Cardinal Joseph Ratzinger, CDF, DECLARATION "DOMINUS IESUS" (August 2000): “…truth of faith does not lessen the sincere respect which the Church has for the religions of the world, but at the same time, it rules out, in a radical way, that mentality of indifferentism “characterized by a religious relativism which leads to the belief that 'one religion is as good as another'”. If it is true that the followers of other religions can receive divine grace, it is also certain that objectively speaking they are in a GRAVELY DEFICIENT situation in comparison with those who, in the Church, have the fullness of the means of salvation.”
---Michael Davies: Without realizing it, they believe in a Church which can fail — and such a Church is not the Church founded by Our Lord Jesus Christ. The Church that He founded cannot fail, for it is indefectible (i.e. it cannot fail). It will continue to exist until the Second Coming as a visible, hierarchically governed body, teaching the truth and sanctifying its members with indubitably valid sacraments.
--Lumen Gentium, Second Vatican Council (1965): Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism and thereby affirmed also the necessity of the Church, for through baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved.
---Canon 1323, 4° explicitly states that necessity exempts a person from penalty only if the act performed is not intrinsically evil or harmful to souls. But the Catholic Church ruled in many Magisterium Documents such as Pastor Aeternus and similar that creating a schismatic parallel hierarchy and directly violating an express, written prohibition from the Pope is recognized by doctrine as inherently harmful to the unity of the Church. Furthermore, the Holy See ruled that Lefebvre's act was not an automatic penalty but an imposed penalty via explicit decree, rendering the "subjective necessity" defenses of Canon 1324 irrelevant.
The SSPX relies heavily on Canon 1752, which states that "the salvation of souls... must always be the supreme law in the Church." However, the Pope, many Cardinals and the Catholic Church as a whole point out that their application of this principle undermines Catholic ecclesiology. The SSPX treats the "supreme law" as a trapdoor that invalidates all other positive laws in the Code.