The phrase also refers to the teaching of the gospel to those in the world of the spirits. “Let the dead speak forth anthems of eternal praise to the King Immanuel, who hath ordained, before the world was, that which would enable us to redeem them out of their prison; for the prisoners shall go free” (D&C 128:22).
While his body lay in a borrowed tomb, Christ in the world of the spirits preached to the righteous dead in paradise. From among them “he organized his forces and appointed messengers, clothed with power and authority, and commissioned them to go forth and carry the light of the gospel to them that were in darkness, even to all the spirits of men; and thus was the gospel preached to the dead. And the chosen messengers went forth to declare the acceptable day of the Lord and proclaim liberty to the captives who were bound, even unto all who would repent of their sins and receive the gospel.” (D&C 138:30-31.)
Thus was Isaiah’s prophecy fulfilled that liberty would be proclaimed to the captives and the prison be opened to them that were bound (D&C 138:42).