The only way going through the barrier was possible was if you had a monster SOUL and a human SOUL inside your body at the same time. And the only way to break the barrier was to have the power of seven human SOULs in you, or its equivalence.

But if you're so interested in the lore...

[Flowey goes on to write information about the monster that made the prophecy, when they made it, where they made it, and how many others felt about the prophecy, listing each individual by name.]