Cardinal Pell gave an interesting perspective from the Modernist standpoint on the SSPX reconciliation with Rome.

Since Vatican II, the civil governments, and also Catholic ones, have recognized no profession of religious belief. The ultimate goal of which is a society of diversity and tolerance. The civil government would surpass its authority if it seeks to enforce a religious belief. This is the teaching of Vatican II.

But let’s take a closer look at the teaching of previous Pope(s):

Pope Pius IX, Syllabus of Errors, Dec. 8, 1864, # 77: “In this age of ours it is no longer expedient that the Catholic religion should be the only religion of the state, to the exclusion of all other cults whatsoever.” – Condemned.

This propositional statement has been condemned by the Catholic Church. It suffices to say that truth should always reign in civil society as well as in religious institutions, that could only mean the Catholic faith, which has no error in it, against all false and man-made religions, should be allowed. Hence, the Catholic faith should not be excluded from the religion of the state, although it was not always considered as such in secular states. There is absolutely nothing evil or untrue about the Catholic faith, neither should the social kingship of Christ be limited to the confines of religious institutions. The Catholic faith and religion is for the good of all men, meant to save all, not just for Catholics. However, the non-Catholics should make the effort to convert for the salvation of their souls and peace, freedom, true human liberty and prosperity in society.