At this point, Jacob is clearly writing as though he were speaking to an audience. It may still be conceptually a future audience, but I suspect that he is transcribing a sermon that he also gave to his own people.
Jacob tells his audience to “marvel not” over the Atoning Messiah’s mission. Of course that mission is marvelous, but it is not a new teaching for Jacob’s people. It has been an essential aspect of their religion from Lehi’s time. Therefore they might marvel at the mission but not that it should be taught.
Nevertheless, the fact that Jacob must teach this sermon indicates that the Messiah’s mission had become an issue among the people. Given my hypothesis that Laman and Lemuel retained a belief in the Josian reform that apparently removed the understanding of the Atoning Messiah and left only that of the Triumphant Messiah, I propose here that the threat of Nephite apostasy during Jacob’s time did not take the form of paganism, but rather one that maintained the law of Moses but denied the atonement as fulfilling that law. Much later in the Book of Mormon this type of apostate Nephite religion will become much more prevalent and divisive in Nephite society. (See 1 Nephi, Part 1: Chapter 1, “The Historical Setting of 1 Nephi, and “Excursus: Religion of the Nehors,” following Alma 1.)