Fr. John Lamont's "Theological Questions" does raise an excellent point: the SSPX and others do argue their position on - for example - the relationship between Church and State/religious liberty - references to previous papal teaching. To this effect, I am posting some key passages from Pius XI's Quas Primas. Interestingly, this encyclical was not referenced by Vatican II, but is footnoted in the Catechism of the Catholic Church (this raises the question whether the framers of the Catechism began to take into account years of criticism of Dignitatis Humanae? and therefore began to nuance the Conciliar declaration to try to show a continuity with previous teaching (e.g. Quanta Cura, Pius IX, Immortale Dei, Libertas, Longinqua, Leo XIII etc.) Archbishop Lefebvre himself argued that the declaration contradicted itself by maintaining the traditional doctrine, yet in its practical advice leading to a de facto religious indifferentism (c.f. They Have Uncrowned Him).
Pius XI, Quas Primas (1925):
It would be a grave error, on the other hand, to say that Christ has no authority whatever in civil affairs, since, by virtue of the absolute empire over all creatures committed to him by the Father, all things are in his power...
If, therefore, the rulers of nations wish to preserve their authority, to promote and increase the prosperity of their countries, they will not neglect the public duty of reverence and obedience to the rule of Christ. What We said at the beginning of Our Pontificate concerning the decline of public authority, and the lack of respect for the same, is equally true at the present day. "With God and Jesus Christ," we said, "excluded from political life, with authority derived not from God but from man, the very basis of that authority has been taken away, because the chief reason of the distinction between ruler and subject has been eliminated. The result is that human society is tottering to its fall, because it has no longer a secure and solid foundation."[ Ubi Arcano, Pius XI]
When once men recognize, both in private and in public life, that Christ is King, society will at last receive the great blessings of real liberty, well-ordered discipline, peace and harmony.
...We refer to the plague of anti-clericalism, its errors and impious activities. This evil spirit, as you are well aware, Venerable Brethren, has not come into being in one day; it has long lurked beneath the surface. The empire of Christ over all nations was rejected. The right which the Church has from Christ himself, to teach mankind, to make laws, to govern peoples in all that pertains to their eternal salvation, that right was denied. Then gradually the religion of Christ came to be likened to false religions and to be placed ignominiously on the same level with them. It was then put under the power of the state and tolerated more or less at the whim of princes and rulers. Some men went even further, and wished to set up in the place of God's religion a natural religion consisting in some instinctive affection of the heart. There were even some nations who thought they could dispense with God, and that their religion should consist in impiety and the neglect of God. The rebellion of individuals and states against the authority of Christ has produced deplorable consequences.