The potential doom of a flesh that would rise no more required a quick resolution. Jacob gives it by underscoring God’s mercy and grace. However, he does return to painting the dark picture of a world without the Atonement by noting that should we not be resurrected, our souls would be subject to the angel who fell. That angel is separated from God, and so humankind would also remain separated from God. Without Jehovah’s laws of light, we would not see our way to become like Him.
The mention of the devil in Jacob’s sermon is technically anachronistic. There was no devil as was understood in later Christian tradition when Lehi’s family lived in Jerusalem. However, since this is a translation, the use of the word that we have come to understand as representing the opposite of God is not inappropriate. Jacob is setting a black and white picture. The devil is black and God white, and that absolute opposite contrast suggests a person opposite God whose realm is also the opposite of God’s. Where Isaiah associated Jehovah’s laws with light, Jacob associates the devil’s realm as one of darkness.
As with other absolute dichotomies in the scripture, we are told that we will eventually choose one or the other. Jacob is noting that without the Atonement, we would not have had the choice of God’s light of the devil’s darkness.