Nephi acclaims Yahweh’s knowledge, then praises the Atoning Messiah’s voluntary suffering on behalf of humankind. The two verses should be read as a single passage, because verse 21 specifies what Yahweh knows (v. 20). Jacob is not interrupting his sermon on the atonement with an aside on Yahweh’s omniscience. Rather, he is linking Yahweh’s knowledge with the event of the atonement. A modern parallel is Doctrine and Covenants 88:6: “He that ascended up on high, as also he descended below all things, in that he comprehended all things, that he might be in all and through all things, the light of truth.”
Both Jacob and Joseph Smith connect a particular kind of knowledge to the necessity for the atonement, an act that apparently was not possible had the Atoning Messiah not so intimately known us.