Christ Fulfils the Law of Moses
"Behold, I say unto you that the law is fulfilled that was given unto Moses." (3 Nephi 15:4)
"Behold, I do not destroy the prophets, for as many as have not been fulfilled in me, verify I say unto you, shall be fulfilled." (3 Nephi 15:6) To me, this verse means that the prophets understood the whole plan of salvation and they understood that the Law of Moses would lead the people to the true law … Christ. So it is understandable that just because Christ fulfilled the Law of Moses that it does not mean all things have been completed. There are many things that must come to pass before the great plan is complete.
Jesus continues, "the covenant which I have made with my people is not all fulfilled." (3 Nephi 15:8)
Look unto me and endure to the end
In His mortal ministry, Christ gave the parable of the sower to teach us that we must endure to the end. (Mark 4:2-14).
What must we endure? We must endure the trials, the temptations of the devil, the sacrifices required of us, the service and the doctrine.
Some lose faith when loved ones suffer. Others give in to temptation. Other members will complain of the sacrifices required of the Lord. Service never ends. Some will complain about how service is never-ending. As Elder Ballard once said, "No matter what your family needs are or your responsibilities in the Church, there is no such thing as 'done.'" (M. Russell Ballard, “O Be Wise,” Ensign, Nov 2006, 17–20). Lastly, there are those who will become offended by a bishop or doctrine. These too lose faith. There are so many ways that we can fall short, but we must endure. I am reminded of King Benjamin's last words, "And finally, I cannot tell you all the things whereby ye may commit sin; for there are divers ways and means, even so many that I cannot number them."
"But this much I can tell you, that if ye do not watch yourselves, and your thoughts, and your words, and your deeds, and observe the commandments of God, and continue in the faith of what ye have heard concerning the coming of our Lord, even unto the end of your lives, ye must perish. And now, O man, remember, and perish not." (Mosiah 4:29-30)
The Lord has many folds. The Bible teaches us of one fold. The Book of Mormon teaches us of another. And still there are more folds. Someday we will not only have to carry to church the Bible, the Book of Mormon and D&C, but we will carry with us the scriptures of the Lost tribes of Israel.
"And it shall come to pass that the Jews shall have the words of the Nephites, and the Nephites shall have the words of the Jews; and the Nephites and the Jews shall have the words of the lost tribes of Israel; and the lost tribes of Israel shall have the words of the Nephites and the Jews." (2 Nephi 29:13)
The bringing forth of new scripture will be preceded by the return of the lost ten tribes. I found an article by an LDS member who wrote about this. He does a good job summarizing the scriptures that foretell this great event. (See article entitled "A Volcanic Highway for the Lost Tribes?" by John P. Pratt - http://www.meridianmagazine.com/sci_rel/050915volcano.html)
To summarize in one sentence, it seems that the return of the lost tribes will be a miraculous event that will be even bigger than when Israel was led out of Egypt