Following on from my post about possible dangers to sound Liturgy arising from PF's own personal liturgical fads and his dirigiste instincts, I want to draw to the attention of readers two loci of Magisterial status. (I presume that readers are already familiar with what the then-Cardinal Ratzinger wrote in The Spirit of the Liturgy, when he criticised the hyperpapalism which, after Vatican II, played on an erroneous assumption that the pope can do anything. This, of course, could be argued to be non-Magisterial.)
The two places that I wish, very briefly, to draw to your attention are full exercises of a Papal Magisterium.
(1) In the Letter to the Bishops which accompanied Summorum Pontificum, pope Benedict XVI wrote "What earlier generations held as sacred, remains sacred for us too, and it cannot be all of a sudden forbidden". Notice the expression cannot. The learned pontiff says, not "should not be"; he says "cannot be".
(2) I suspect Ratzinger of being responsible for drafting paragraph 1125 of the Catechism of the Catholic Church, although, of course, it was promulgated with the force of an Apostolic Constitution in 1992 by Pope S John Paul II.
The second sentence of this paragraph begins with the phrase "Ipsa auctoritas Ecclesiae suprema [Even the supreme authority of the Church itself]". This is a phrase commonly used, especially at Vatican II, of the Pope himself (although surely it would also apply to an Ecumenical Council). Next comes "non potest [is not able]". I ask you to notice that we do not have "non licet" ["he is not permitted"], nor do we have a jussive subjunctive ["he shouldn't do it"]. What is being excluded is being excluded as an impossibility. Just as S John Paul II excluded the sacerdotal ordination of women as an impossibility (nullam facultatem habere).
The sentence in the Catechism continues: "liturgiam ad placitum commutare suum [change the Liturgy in accordance with his own fads] sed solummodo in oboedientia fidei et in religiosa mysterii liturgiae observantia [but only in the obedience of the Faith and in the religious observance of the mystery of the Liturgy]."
In other words, if a pope were to attempt to change the Liturgy in accordance with his personal fads, he would be acting ultra vires. And so his attempt would be null.
I suspect we would have to go back to the principled and glorious teaching of Vatican I (Pastor aeternus) to find as clear and forthright a Magisterial statement of what a pope is not competent to do!
My apologies to readers who recognise this as the theme of a paper I have read in a number of places during the last five years, sometimes under the title "Can the pope abolish the Vetus Ordo?". I am willing to dig it out and update it and come and read it again if anyone else wants to hear it!!! Anywhere in Europe! Just my expenses!