1 Ne. 10: 11 And it came to pass after my father had spoken these words he spake unto my brethren concerning the gospel which should be preached among the Jews, and also concerning the dwindling of the Jews in unbelief. And after they had slain the Messiah, who should come, and after he had been slain he should rise from the dead, and should make himself manifest, by the Holy Ghost, unto the Gentiles."
Is this talking about a physical manifestation to the Gentiles? Does Christ show Himself to others physically by the power of the Holy Ghost? Or is this to the Gentiles' hearts and minds before the Restoration?
This is speaking about the immediate post-resurrection ministry of the Lord. At that time He visited only with the tribes of Israel in their scattered condition. He did not go among the gentiles. Nephi explained that in the last days ministry of the Lord at that time, He would appear to the gentiles "in very deed." (1 Ne. 14: 1.) This is why the Lord appeared to Joseph Smith (JS-H 1: 17-19) and Oliver Cowdery (D&C 110), and to Sidney Rigdon (D&C 76), and to others.
In Alma 13, the prophet concludes his testimony of Melchizedek by using a sign to evidence his authority. He used this sign because he was authorized to do so, and knew what it meant as he did. Although those who were there may not have understood, it was a sign he was a true messenger.