The first thing to remember is that it’s happened before, according to the Bible.
Assumption is not the same phenomenon as Ascension. Ascension is rising under one’s own power. Jesus alone “ascended” into heaven by His own power, “No one has ascended into heaven but he who descended from heaven, the Son of man,” John 3:13. Assumption is the passive event of being taken by God. Scripture shows that Enoch, Elijah, and Moses were “assumed” or taken bodily into heaven, which is what Catholics believe happened also to Mary.
“Thus all the days of Enoch were three hundred and sixty-five years. Enoch walked with God; and he was not, for God took him,” Gen. 5:23-24.
“And as they still went on and talked, behold, a chariot of fire and horses of fire separated the two of them. And Elijah went up by a whirlwind into heaven.” 2 Kings 2:11.
“But when the archangel Michael, contending with the devil, disputed about the body of Moses, he did not presume to pronounce a reviling judgment upon him, but said, ‘The Lord rebuke you,’” Jude 9.
Notice that Assumption does not mean that the person did not die. Enoch and Elijah were assumed while still alive, but Moses, in contrast, had already died when his body was assumed into heaven. Assumption therefore can be of either a living person or a dead body.
The Catholic Church takes no position on whether or not Mary died. Munificentissimus Deus, Pope Pius XII’s ex cathedra statement made on November 1, 1950 which formally defined the doctrine of Mary’s Assumption, states: “…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,” MC 44. Since no position is taken on whether or not Mary died, Catholics may believe that she, like Enoch and Elijah, was assumed without death, or that she died and her body was assumed, like that of Moses. Either way, Biblical precedent shows that Mary was not the first person to have been granted this blessing.