Well, you need to tell the Popes, catholic.com, newwordencyclopedia and many others of that because they all say it is.
Pope Pius XII solemnly defined in Munificentissimus Deus on Nov. 1, 1950, 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."
The doctrine of the Assumption says that at the end of her life on earth Mary was assumed, body and soul, into heaven, just as Enoch, Elijah, and perhaps others had been before her. It’s also necessary to keep in mind what the Assumption is not. Some people think Catholics believe Mary "ascended" into heaven. That’s not correct. Christ, by his own power, ascended into heaven. Mary was assumed or taken up into heaven by God. She didn’t do it under her own power.
The Church has never formally defined whether she died or not, and the integrity of the doctrine of the Assumption would not be impaired if she did not in fact die, but the almost universal consensus is that she did die. Pope Pius XII, in Munificentissimus Deus (1950), defined that Mary, "after the completion of her earthly life" (note the silence regarding her death), "was assumed body and soul into the glory of heaven." [http://www.catholic.com/library/Immaculate_Conception_and_Assum.asp]
Although the doctrine of the Assumption of Mary may somewhat controversially [http://www.newworldencyclopedia.org/entry/Assumption_of_Mary]
The wasn’t part of any church untill the 5th century. It’s absent in scripture.
“The belief in the corporeal assumption of Mary is founded on the apocryphal treatise De Obitu S. Dominae, bearing the name of St. John, which belongs however to the fourth or fifth century. It is also found in the book De Transitu Virginis, falsely ascribed to St. Melito of Sardis, and in a spurious letter attributed to St. Denis the Areopagite” (Catholic Encyclopaedia).
The first church author to speak on the assumption, Gregory of Tours, based his teaching on the Transitus, perhaps because he accepted it as genuine. However, in 459 A.D. Pope Gelasius issued a decree that officially condemned and rejected the Transitus along with several other heretical writings. Pope Hormisdas reaffirmed this decree in the sixth century. It is ironic that this heretical teaching was later promoted within the Catholic Church, until eventually it was proclaimed a dogma in the twentieth century.
Looks like you really need to believe also or be considered “fallen away”.
The Roman Church solemnly warns anyone who “should dare willfully to deny or to call into doubt that which we have defined (i.e. the Assumption), let him know that he has fallen away completely from the divine and Catholic Faith (Munificentissimus Deus).
The apostates are the ones who have invented this doctrine. The faithful are those who, together with the early Christians, have remained steadfast in upholding the faith of the New Testament.
I think you Catholics had better do some intense study of what your Catholic Church used to be and how far they have fallen away. Notice how the whold concept was based on writings that were origianlly declared haresy.
No, it isn't. De Transitu is only one source that speaks of the Assumption, and it wasn't "declared haresy" because of its teaching of the Assumption, but for other reasons.
I fail to see why you would find the Assumption such an intolerable doctrine, though. We think Mary was raptured on the early flight. So what?