"Behold, I do not destroy the prophets, for as many as have not been fulfilled in me, verily I say unto you, shall all be fulfilled. And because I said unto you that old things have passed away, I do not destroy that which hath been spoken concerning things which are to come. For behold, the covenant which I have made with my people is not all fulfilled; but the law which was given unto Moses hath an end in me."
The Lord does not make a promise and fail to fulfill it. (D&C 1: 38.) Therefore, when a promise has been made by Him, it will come to pass. But the promise must be His. No agent or spokesman can speak in His name and obligate Him to perform unless the words spoken are His. Even if a man should qualify to hold sealing power, that power will only bind what is in conformity with His word. (Helaman 10: 5.) There is no obligation on Him to perform what is not sealed by the Holy Spirit of Promise. (See, e.g., D&C 132: 18 and D&C 88: 3.) So it is not every person who speaks, even if in a position of leading others, claiming "Lord, Lord" as they do, whose words obligate the Lord to fulfill. But the opposite is also true. If the person is clothed with nothing other than the Lord's private commission to speak, if he speaks the Lord's words they will "all be fulfilled." Abinadi was so obscure a character that we don't know if he was Lamanite or Nephite. He is the only person in the entire Book of Mormon record with the name Abinadi. He came from nowhere, was imprisoned by the leading authorities of the church, and was killed by those who presumed to exercise judgment over him. Yet it was he who bore the Lord's words. The entire society he preached to were held to account for both his words and how they reacted to them (and him).
The intergenerational work of saving mankind is always the same. The promise to save through the chosen lineage all of mankind is still in effect. It existed before Moses, and will continue after the fulfillment of the law of Moses. The great prophecies and promises pointing to His second coming remain in effect. His first coming only fulfilled Moses' law. His second will fulfill the rest of the promises concerning Him as the great Deliverer, the world's judge, and the one whose right it is to rule as "King of kings and Lord of lords."