According to tradition, the Apostle Luke got the story of the Nativity from Mary, herself. Is it out of the question to assume that he and some of the other Apostles may have witnessed Mary's Assumption and told the followers of Jesus about it? If it's not in the Bible, that wouldn't be unusual, after all, the Gospels are the story of Jesus's Incarnation, His public life of ministry, His Crucifixion, and His Resurrection. The letters from Paul, Peter and the other apostles to the new Christian communties were exhortations to live as Jesus had taught.
The New Testament is not Mary's story. That's why it doesn't include stories of what she did when she left Jerusalem, after Jesus's Ascension. We know Jesus placed her in the care of the youngest Apostle, and there are suggestions that she eventually moved to Ephesus, where there was a new Christian community. There is a house there that is purported to be that of Mary. But that isn't in the New Testament, either, even though Paul writes to the Ephesians several times.
I choose to believe that it is tradition, passed down through the teachings of the Apostles, and later their disciples. You may not believe it, but I don't know of any evidence that it isn't true.
According to tradition, John Smith received the Book of Mormon from the Angel Moroni, himself.
I choose to believe that it is tradition, passed down through the teachings of the Apostles, and later their disciples. You may not believe it, but I don't know of any evidence that it isn't true.
Just like there's no evidence that the lost Plates of Nephi and the Book of Mormon aren't true either.
The Bible teaches that all humans are sinners. I don't think the apostles would have passed on by word of mouth the doctrine of the so-called immaculate conception since it so dramatically contradicts their writings in the New Testament which emphasize over and over that all men sin. Why would anyone believe anything they had to say if they contradicted themselves in that way? This is why Protestants can't believe the doctrine...not so much that it is based on supposed verbal tradition, but that it is overtly against the apostles' Biblical writings.
For example, why would John in Ephesus write in his epistle in 1 John 1:10 that "if we claim we have not sinned we make him out to be a liar..." if there was actually one who was living or had lived among them in Ephesus (Mary) who supposedly had not sinned?
Or why would Paul write, "For all have sinned and fall short of the glory of God" if he knew Mary was not a sinner? In order to make that not a falsehood he would have had to have added "for all have sinned except Mary". Paul especially says over and over and over again that all of humankind except Christ is in a lost condition because of sin. I take him at his word.