In this future world grown apart from their God, and yet still believing, they must come to a reconciliation of their professed belief and the evident withdrawal of their God. Nephi is describing a situation that must be conceptually foreign to him. Nephi, as a Jew, traces his heritage to a God who was physically present in the pillar of the cloud and the pillar of light which guided the Exodus (Ex. 13:21).
For Nephi, the condition of being without his God would be the definition of foreign - a people who were without benefit of the covenant. Thus this future gentile population, and the wayward Jews, were both in a state foreign to the covenant. They will be without their God.
Nevertheless, a people continuing to profess faith in God, yet not having that God with them, will have to have ways of justifying his absence. Nephi predicts that they will justify the absence of the active presence of their God on the basis of the accomplishment of the Atonement. Having this great event in the past, they will justify the lack of current miracles, the lack of a currently present God, to the finality of that atoning event.