In discussions of whether or not the pope has the authority to radically change the Church’s liturgy, the claim has been made that “if he can modify the bread from leavened to unleavened and limit reception to one species—which pertain even to the substance of the sacrament!—then of course he can do anything else.” I want to explain why this claim is a gigantic non sequitur.
Before proceeding, I will note that there is no good historical evidence that all ancient liturgies were using leavened bread until at some point a pope flexed his pontifical muscle to say: “No more! From here on out, we will use only unleavened bread.” The comprehensive article on “Azymes” by Church historian Dom Fernand Cabrol in the authoritative Dictionnaire d’Archéologie Chrétienne et de Liturgie (vol. 1, cols. 3254–60) says nothing about such a purported papal change. We need to be careful not to fall into the trap of the common assumption (which in the field of liturgy is totally unjustified) that everything in the East is much more ancient than everything in the West, and that the East never changes; ergo, if the East uses leavened bread, and the West uses unleavened bread, that must be because the older and “original” custom is to use leavened bread, and the West must have changed it. In any case, evidence does not support the idea that any shift that may have occurred was due to simple papal fiat.[1] Early Church history is generally a lot more complicated, confusing, and polymorphic—and, it’s always humbling to remember, a lot less documented—than we, with our tidy scholastic boxes, might wish it were.
Be that as it may, let’s assume, for the sake of argument, that a pope could make such a change of matter in the sacrament, and that he did make it. Similarly, he could choose to permit or not permit Communion under both kinds; he may add or not add the Filioque; and any number of other discrete changes of that sort. The interesting question is: How much does it matter, and what does it prove?