3 Nephi 15: 6-8:
"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).
"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).
When the Lord speaks of fulfilling the things to come, He is both ratifying the past prophets whose words have not come to pass, and He is establishing an eternal principal. It is as true today as it was anciently. When a message comes from Him, it is binding. The message is His. The power to make His message binding upon mankind is His. The right to govern all mankind is His.
The first clarification the Lord wants the people to understand is that His words are, and will remain sovereign. They will not be rescinded. It is not the prophets, nor the promises of His great unfolding work foretold by prophetic messages that will end. It is only the law of observances given through Moses that has now been fulfilled. It is not abandoned, but rather it is fulfilled. It pointed to Him. He lived it. He fulfilled every foreshadow, every type, every promise under that law. It was His to give, and it was His life that fulfilled it.
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."
The crowd entertained apprehensions that the prophets were now "destroyed" by Him. He made it clear that was not the case. This is why Isaiah and Zenock remain relevant to our day. This is why He will even quote from Isaiah and add Malachi to the Nephite scriptures. This is why the Lord continues to entrust men with messages which bind Him to do His final, strange work. He intends to both fulfill and inform us so we may prepare against the day of judgment. His mission is to redeem, not to surprise or confuse the worthy. If a person will but listen to Him and those He sends, they will be prepared for the coming calamities.
The consistency of this message is so profound that it reconfirms that Joseph Smith is not the source of the Book of Mormon. This is a record of the Lord's doings among an ancient and fallen people. It is not an invention of a New England farm boy. The idea Joseph Smith wrote this account is beyond incredible. It simply isn't true. This is from the Lord, not a man. A person can get closer to God by abiding its precepts than from any other book. It is the lifeline given to us for our day. We ignore it and dismiss it at our peril.