I think others are doing a good job there, but "the more the merrier", so I'll put in my $00.02 worth too, since attempting some sort of answer to "why do Catholics believe so-and-so?" or "how do you support that from the Bible?" is what apologetics is about, and I do apologetics for a living. Besides, it isn't often that one runs across someone who is vigorously asking about important questions that divide Protestants and Catholics. It's a great opportunity for constructive discussion. His words will be in blue.
First of all, it is an assumption, not an "assent" (proper spelling: ascent). Jesus ascended to heaven on His own power. Mary was bodily assumed by God's power. The Assumption is not explicitly in Scripture, but neither is sola Scriptura: yet you are willing to base your entire structure of authority based on that mythical man-made tradition. On the other hand, it is not contrary to anything in Scripture. The Assumption is simply the first instance of resurrection: something all believers will one day attain. I wrote about it in my book, A Biblical Defense of Catholicism:
Another example of a biblical "assumption" is that of the Two witnesses in Revelation:
Lest one think that a bodily ascent to heaven (of a creature, as opposed to Jesus) is impossible and "biblically unthinkable," Holy Scripture contains the examples of Enoch (Hebrews 11:5; cf. Genesis 5:24), Elijah (2 Kings 2:1,11), St. Paul's being caught up to the third heaven (2 Corinthians 12:2-4), possibly bodily, and events during the Second Coming (1 Thessalonians 4:15-17), believed by many evangelicals to constitute the "Rapture," an additional return of Christ for believers only. All these occur by virtue of the power of God, not the intrinsic ability of the persons.
The Assumption of the Blessed Virgin flows of necessity from the Immaculate Conception and Mary's actual sinlessness. Bodily death and decay are the result of sin and the Fall (Genesis 3:19, Psalm 16:10). Thus, the absence of actual and original sin "breaks the chain" and allows for instant bodily resurrection and also immortality, just as God intended for all human beings.
. . . Jesus' Resurrection brings forth the possibility of universal resurrection (1 Corinthians 15:13,16), which is why He is called the "first fruits" (1 Corinthians 15:20-23). Mary's Assumption is the "first fruits," sign, and type of the general resurrection of all mankind, so that she represents the age to come, in which death and sin will be conquered once and for all (1 Corinthians 15:26). The Assumption is, therefore, directly the result of Christ's own victory over sin and death. It, too, has a Christocentric meaning, in the same way as the Immaculate Conception and the designation Theotokos.
Revelation 11:11-12: But after the three and a half days a breath of life from God entered them, and they stood up on their feet, and great fear fell on those who saw them. Then they heard a loud voice from heaven saying to them, "Come up hither!" And in the sight of their foes they went up to heaven in a cloud.Please demonstrate in Scripture where it is written that Mary was assumed, was sinless and was to be prayed to? Because no one has been able to show it so far.
. . . How does Mary being called highly favored and blessed among women equate to meaning she is sinless despite the Bible distinctly saying that all have sinned?
The Assumption is not explicitly stated, but is not contrary to anything in Scripture. Her sinlessness is expressly stated in Luke 1:28 ("full of grace"). I have made extensive analyses of this:
Dialogue on the Exegesis of Luke 1:28 ("Full of Grace"), and the Immaculate Conception (vs. Ken Temple)
This constitutes an explicit biblical proof. Catholic theology then reflected upon what this means, and concluded that Mary would not decay when she left this earth, and so the Assumption followed from that. It was an instant resurrection.
I dealt with the "all have sinned" argument in another paper.