10 For We can see that some of those who are dealing with this Most Holy Mystery in speech and writing are disseminating opinions on Masses celebrated in private or on the dogma of transubstantiation that are disturbing the minds of the faithful and causing them no small measure of confusion about matters of faith, just as if it were all right for someone to take doctrine that has already been defined by the Church and consign it to oblivion or else interpret it in such a way as to weaken the genuine meaning of the words or the recognized force of the concepts involved.
Source: Mysterium Fidei (Vatican.va)