When Alma begins by saying that “you marvel why these things should be known so long beforehand,” he lets us know that Corianton has fallen into the common apostate arguments that anti-Messiah preachers have been using since Sherem by saying that one cannot know that Christ will come; it is too far in the future. Alma will work to show Corianton that this isn’t the strong argument that he had thought it to be.
At the root of this question will be the nature of prophecy and communication with Jehovah. Alma begins by setting the stage by suggesting that since the atonement is so important, should God not make it known? Doesn’t God have a responsibility to let his children know that the power of redemption will be manifest? Because it is in the future, God can do this only be sending “his angel to declare these glad tidings unto us as unto our children.”
There is no chapter break at this point in the 1830 edition. There is a shift to a slightly different topic. Alma begins by explaining that it is right that God should announce the atoning mission of the Messiah. He will now move to explain the importance of that atoning mission. Because that created a shift in the topic, Orson Pratt separated it into a different chapter.