Victor R. Claveau, MJ
Answer: The strongest proof for the Assumption according to “Munificentissimus Deus” is the unanimous faith of the people and their pastors. This remarkable accord of the Catholic bishops and faithful enlightened by the Holy Spirit existed in the Church for centuries. In a matter of so great importance that Church cannot make a mistake or be deceived for the Lord himself promised to be with the Church until the end of time. Pope Pius XII, therefore, did not receive or claim to receive a new revelation from above, nor did he invent a new doctrine of the Church. He simply declared in his definition what the faithful and the bishops had already believed. In a word, he was dependent on the lived faith of the people. It would be more accurate to say the Pope defined the Assumption because Catholics believed in it than to say Catholics believed in it because the Pope defined it.
For further information, I would recommend “Munificentissimus Deus” also, “The Assumption of Mary” by Killian Healy, O. Carm. “Queen of the Universe, An Anthology on the Assumption and Queenship of Mary”, Mathews, Stanley. Grail 1957 St. Meinrad Indiana; and “The Glorious Assumption of the Mother of God”,Duhr, Joseph, S.J., NY: P.J. Kennedy & Sons c. 1950.