From Messa in Latino (trans. Rorate Caeli, since removed):
The content of the Preamble, which at the end of the day is a somewhat synthetic document, can be essentially summed up in two points. We begin with the second one because it is a simple thing: to say it in broad terms, the FSSPX must modify its tone and express whatever it wishes to say in a respectful and filial way, as well as collaborate loyally with all the other structures of the Mystical Body. In clerical-theological level, that is defined as 'sentire cum ecclesia'.
The first point of the preamble, instead, the most important one, is the re-proposition of the content of canon 750 of the CIC [Code of Canon Law], that is, the need for a Catholic to accept the magisterial teaching according to the degrees of assent sanctioned by that canon and by the Apostolic Letter Ad tuendam fidem, of John Paul II. To sum it up, there are diverse degrees of binding of magisterial teaching: as an Explanatory note of then-Cardinal Ratzinger, as Prefect of the Congregation for the Doctrine of the Faith, clarified, there are truths that the Church proclaims as divinely revealed, and that are thus unchanging, and to be received with "theological faith". Whoever does not believe in it is not a Catholic. Such are, for instance, the dogmas of faith, regarding which, in any event, the SSPX has no problem (Bishop Fellay exemplified it with the dogma of the Trinity). An equal assent of firm faith (and equally non-problematic for the FSSPX) concerns those doctrines on faith and morals not directly founded on Scripture, but taught by the Church infallibly, either because proclaimed thus or because always repeated by the Magisterium. Examples of the latter (read in the explanatory note itself) include the impossibility of female ordination, the prohibition of euthanasia, the canonization of saints.
Those teachings of the Magisterium of the [Supreme] Pontiff or of the College of Bishops which require mere 'religious assent of mind and intellect' are those that are not definitive (perhaps because they contradict preceding teachings: one could consider - the example is ours [Messa in latino's] - the prohibition of loans with interest). The Note of the Congregation for the Doctrine of the Faith prudently avoids mentioning examples of any kind, perhaps because it would be as if minimizing the teachings that might have been listed in this category. The fact is that the most controversial teachings of the Council, as also [of] the subsequent Magisterium that has repeated those teachings, could not rise up ... to a binding level above this one, considering that the Council declared its will not to define any new "truth" and that the very fact of being propositions, if not in "rupture", at least "reforming" the previous Magisterium, they are necessarily deprived of any character of definitiveness.
In practice, it is asked of the Fraternity to sign the profession of faith which every Catholic must hold; it seems pretty feasible. But some could fear that this obligation of "religious assent of mind and intellect", if applied to certain Conciliar teachings, could curtail, even if it would not nullify (under certain conditions, it is possible to dissent - but not loudly - from non-definitive teachings), the right of criticism to the Council. And here is the great innovation.
As the official communiqué of the Holy See reports, the Preamble leaves "open to legitimate discussion the study and theological explanation of particular expressions and formulations present in the texts of the Second Vatican Council and of the Magisterium that followed it." Let it be noted that the object of this discussion, which is expressly recognized as "legitimate", is not merely the interpretations of the documents, but the very texts of the latter: the "expressions or formulations" used in the conciliar documents. If the words used in the preamble, and thus in the official communiqué, have a sense, there is here a Copernican revolution in the approach to the Council: that is, the displacement from a mere exegetical level to a substantive one.