In 1950, on the day after Reformation Day, Pope Pius XII vindicated the Reformation in his decree Munificentissimus Deus. The Pope, after referring to the bodily assumption of Mary as "a matter of such great moment and of such importance", goes on to make comments like the following, which reflect how weak the case is for this doctrine he was about to bind upon all of "the faithful":
"However, since the liturgy of the Church does not engender the Catholic faith, but rather springs from it, in such a way that the practices of the sacred worship proceed from the faith as the fruit comes from the tree, it follows that the holy Fathers and the great Doctors, in the homilies and sermons they gave the people on this feast day, did not draw their teaching from the feast itself as from a primary source, but rather they spoke of this doctrine as something already known and accepted by Christ's faithful....When this liturgical feast was being celebrated ever more widely and with ever increasing devotion and piety, the bishops of the Church and its preachers in continually greater numbers considered it their duty openly and clearly to explain the mystery that the feast commemorates, and to explain how it is intimately connected with the other revealed truths....Consequently it seems impossible to think of her, the one who conceived Christ, brought him forth, nursed him with her milk, held him in her arms, and clasped him to her breast, as being apart from him in body, even though not in soul, after this earthly life....Finally, since the Church has never looked for the bodily relics of the Blessed Virgin nor proposed them for the veneration of the people, we have a proof on the order of a sensible experience....Since the universal Church, within which dwells the Spirit of Truth who infallibly directs it toward an ever more perfect knowledge of the revealed truths, has expressed its own belief many times over the course of the centuries, and since the bishops of the entire world are almost unanimously petitioning that the truth of the bodily Assumption of the Blessed Virgin Mary into heaven should be defined as a dogma of divine and Catholic faith - this truth which is based on the Sacred Writings, which is thoroughly rooted in the minds of the faithful, which has been approved in ecclesiastical worship from the most remote times, which is completely in harmony with the other revealed truths, and which has been expounded and explained magnificently in the work, the science, and the wisdom of the theologians - we believe that the moment appointed in the plan of divine providence for the solemn proclamation of this outstanding privilege of the Virgin Mary has already arrived."
The Pope goes on to tell us that he hopes the doctrine will produce among the faithful "a stronger piety toward their heavenly Mother", which supposedly "redounds to the glory of the Most Blessed Trinity, to which the Blessed Mother of God is bound by such singular bonds". Then we're told:
"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 glory. Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith....It is forbidden to any man to change this, our declaration, pronouncement, and definition or, by rash attempt, to oppose and counter it. If any man should presume to make such an attempt, let him know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul."
Thank God for the Reformation!
You can read about some of the historical problems with the Assumption of Mary here and here.
"they who are at Rome do not observe those things in all cases which are handed down from the beginning, and vainly pretend the authority of the apostles...But with respect to the refutation of custom which they [the Roman church] seem to oppose to the truth, who is so foolish as to prefer custom to truth, or when he sees the light, not to forsake the darkness?...And this indeed you Africans are able to say against Stephen [the bishop of Rome], that when you knew the truth you forsook the error of custom. But we join custom to truth, and to the Romans' custom we oppose custom, but the custom of truth; holding from the beginning that which was delivered by Christ and the apostles" (Firmilian, Cyprian's Letter 74:6, 74:19)
"And as if it were too little to have disturbed everything here, it [heresy] introduced a ship freighted with blasphemies into the port of Rome itself. The dish soon found itself a cover; and the muddy feet of heretics fouled the clear waters of the faith of Rome. No wonder that in the streets and in the market places a soothsayer can strike fools on the back or, catching up his cudgel, shatter the teeth of such as carp at him; when such venomous and filthy teaching as this has found at Rome dupes whom it can lead astray." (Jerome, Letter 127:9)