54 After the Council of Trent, Our predecessor, Pius VI, issued a serious warning, on the occasion of the errors of the Synod of Pistoia, that parish priests not neglect to speak of transubstantiation, which is listed among the articles of the faith, in the course of carrying out their office of teaching. (57) Similarly, Our Predecessor of happy memory, Pius XII, recalled the bounds beyond which those who were carrying on subtle discussion of the mystery of transubstantiation might not pass; (58) and We Ourself, at the National Eucharistic Congress that was recently celebrated at Pisa, bore open and solemn witness to the faith of the Church, in fulfillment of Our apostolic duty. (59)
Source: Mysterium Fidei (Vatican.va)