Verse 20 begins with an acclamation of the knowledge of God. This acclamation is followed by the statement of Christ’s voluntary suffering on the behalf of man. The two should be read conjointly, because it is the knowledge in verse 21 that is being introduced in verse 20. Jacob is not breaking his sermon on the atonement to give an aside on the omniscience of God, but is rather linking that knowledge to a specific event. The modern scripture which best parallels this sentiment is found in the Doctrine and Covenants, section 88:
D&C 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 specifically link a particular kind of knowledge to the necessities of the atonement for sin, an act that apparently was not possible had Christ not so intimately “known” us.