Catholic theologians consider the declaration to be an ex cathedra use of Papal Infallibility .
By the authority of our Lord Jesus Christ , of the Blessed Apostles Peter and Paul , and by our own authority , we pronounce , declare , and define it to be a divinely revealed dogma : that the Immaculate Mother of God , the ever Virgin Mary , having completed the course of her earthly life , was assumed body and soul into heavenly gloryConstitution Munificentissimus Deus , no 44 Roman Catholic theologians consider this declaration by Pius XII to be an ex cathedra use of Papal Infallibility.  Fidelity : Weighing and Interpreting Documents of the Magisterium , by Francis A. Sullivan , chapter 6 Although Pope Pius XII deliberately left open the question of whether Mary died before her Assumption , the more common teaching of the early Fathers is that she did die .
Assumption of Mary