"In that day" in this verse refers to the time when the restoration of the plain and precious parts of the Gospel will occur. Christ, identified here as the Lamb of God ( a designation which should be quite powerful to Nephi who understands the symbolism of the lamb sacrifice in the Mosaic law) will manifest himself unto the world "in power, in very deed." While some of the fulfilment of this prophecy may yet await, clearly much of the intent was fulfilled in the events of the restoration of the Gospel, and the visitation of Christ to Joseph Smith.