Jacob quotes Isaiah 50, where Isaiah is speaking messianically. Notice the questions the Lord raises right up front. The first question is new in the Book of Mormon; it is not in the King James Bible. If a man found uncleanness (infidelity) in his wife, he could put her away with a bill of divorcement (see the Mosaic law in Deuteronomy 24:1–4).
Here again we have marriage imagery: God is married to his people, but they had estranged themselves because of their wickedness. Recall the complaint (1 Nephi 21:14; Isaiah 49:14) that the Lord had forsaken and forgotten them—but he would show that he had not. They were separated but not divorced.
The Lord does not forsake us when we sin. We forsake him. He does not “sell” us. We sell ourselves when we decide to give up our eternal souls for the pleasures of the moment (compare Moses 8:15).
It is easy to see why Isaiah used the symbolism of divorce and slavery to describe the relationship between Christ and Israel. When the people of Israel (ancient or modern) commit sin, they are slaves in the “bondage of sin” (D&C 84:49–51). In ancient Israel, idolatry was regarded as spiritual adultery, and Jesus Christ, the Bridegroom, deserves unadulterated fidelity.