But can a Catholic truly do that? What about religious assent?
It's rather complicated, but religious assent is distinguished from "assent without reservation">
http://www.catholicapologetics.info/thechurch/encyclicals/docauthority.htmMsgr. Fenton is speaking here (cited below) about papal encyclicals, but it would apply
a fortiori to less-authoritative expressions of the Magisterium as well. Then, even within Encyclicals, it would depend on whether it appeared as if the pope were attempting to authoritatively teach something or else was just merely expounding on a subject. Certainly, we don't read a papal encyclical as we would Sacred Scripture, as if every word were inspired or inerrant.
Hence it follows that the authority of the encyclicals is not at all the same as that of the solemn definition, the one properly so-called. The definition demands an assent without reservation and makes a formal act of faith obligatory. The case of the encyclical’s authority is not the same.
This authority (of the papal encyclicals) is undoubtedly great. It is, in a sense, sovereign. It is the teaching of the supreme pastor and teacher of the Church. Hence the faithful have a strict obligation to receive this teaching with an infinite respect. A man must not be content simply not to contradict it openly and in a more or less scandalous fashion. An internal mental assent is demanded. It should be received as the teaching sovereignly authorized within the Church.
Ultimately, however, this assent is not the same as the one demanded in the formal act of faith. Strictly speaking, it is possible that this teaching (proposed in the encyclical letter) is subject to error. There are a thousand reasons to believe that it is not. It has probably never been (erroneous), and it is normally certain that it will never be. But, absolutely speaking, it could be, because God does not guarantee it as He guarantees the teaching formulated by way of definition’.
Lercher teaches that the internal assent due to these pronouncements cannot be called certain according to the strictest philosophical meaning of the term. The assent given to such propositions is interpretative condicionatus, including the tacit condition that the teaching is accepted as true “unless the Church should at some time peremptorially define otherwise or unless the decision should be discovered to be erroneous.”
So internal assent is an attitude of "infinite respect," where we give it every benefit of the doubt, assume that it's true/correct unless proven otherwise, and even then we are not permitted "to contradict it openly and in a more or less scandalous fashion." INTERNAL assent, however, means that's it's not enough just to shut up about it while internally thinking "that's a bunch of garbage". It should be a sincere and interior respect and benefit of the doubt and assumption that it's true unless proven otherwise.
This is precisely why I took SeanJohnson to task over his arrogant and disrespectful excoriation of Bergogio's latest "Magisterium." If you believe this is the Vicar of Christ, then it is a grave thing indeed to take that attitude toward his teaching. You accept it with respect and reverence, and only respectfully disagree where it's clearly in error. This haughty attitude of just dismissing anything taught by the man they hold to be the Vicar of Christ is simply not Catholic. That is one of the pernicious fruits of R&R, the erosion of the respect due to the authority of the Magisterium and to the Holy Father.