An angel had asked Nephi if he understood the condescension of God as part of Nephi’s vision of the Tree of Life (see 1 Nephi 11:16). Then, as now in Jacob, the condescension of God is in leaving the heavens in order to be incarnated as a mortal and to make the Atonement. This statement reprises the essentials of Jacob’s sermon about the ability of the Atonement to save us all from death and hell. However, Jacob has more to say on the next day of his sermon, and he makes a brief transition into the topic for the next day.
The Atonement saves us from being ultimately destroyed, but the image of their seed not being destroyed invokes the covenant of the land. Thus, Jacob pivots from the universal effect of the Atonement to the specific salvation of their branch of Israel.
Perhaps there would be a chapter break at this point simply because the sermon continued the following day, and there would be a logical break. While that is possible, logical topical divisions are rarely behind the chapter breaks in the original Book of Mormon. This one is occasioned by the testificatory amen which always forces a chapter break in Nephi’s writings.