When the Lamanite missionaries, accompanied by Nephi and Lehi, go to the land northward, Mormon is picking up an important theme from Helaman 3. He wants his readers to understand that the gospel has been preached in the land northward because destruction will come to his people from the corrupt, apostate, and warlike northern peoples. This detail also suggests that, while some righteous people emigrated north (Hel. 3:12), so did nonbelievers. Otherwise, there would be no need for a mission to convert them.