Nephi, as any prophetic writer, says what "the Spirit hath constrained" him to say. This is the very definition of using the Lord's name with permission and not using His name in vain. (Exo. 20: 7.)
Nephi held power from God in the words he used. Therefore he could "know that they must surely come to pass" because he sealed them as he wrote them. (D&C 1: 38.) For any person holding the sealing authority (which is an indispensable part of the Patriarchal Priesthood discussed earlier), the authority requires an alignment between the prophet, the Lord and the Lord's will. (See, D&C 132: 45-49, in particular verse 48 which mentions "by my word and according to my law"--which required Joseph to align himself with the Lord before using that power.) Those who have this authority will not do anything contrary to the will of the Lord. (Helaman 10: 5.) It is because of this trust between the Lord and His messenger that the power is given to the man. Nephi was such a man. His book contained a seal upon it bearing the power of God.
Nephi knew. Knowledge came from Christ. Nephi knew Christ. (2 Ne.11: 3.)
Notice how Nephi refers to the "remnant" who are "our seed." Nephi refers to the remnant variously as:
-descendants of his father Lehi (1 Ne. 13: 34)
-his family's descendants or "our seed" (1 Ne. 15: 13-14)
-a mixture of Nephi's descendants who are among his brother's descendant's (1 Ne. 13: 30
Nephi's primary line of descendants would be destroyed, but that destruction would not include all. There would remain a mixture of blood that would include partial descent from Nephi. (1 Ne. 13: 30-31) The various bloodlines remained identified as Nephites, Jacobites, Josephites, Zoramites, Lamanites, Lemuelites and Ishmaelites. (Mormon 1: 8.) Although it would be impossible, without revelation, for us to determine which of these lines a person might belong to today, the Lord nevertheless revealed in 1828 that these various divisions remain identified to Him. (D&C 3: 16-19.) No doubt, in time, He will restore to the remnant descendants this knowledge of their sacred paternity and eternal identity.
Their blood may be mixed, but the remnant remains. Nephi may have referred to them more often as descendants of his "brethren," but they have within them some of his blood as well. In the day of redemption and restoration, the promises will all be fulfilled. The whole of the family of Lehi will be represented in the remnant.
Notice Nephi's prophecy is that "words which shall be written out of the book" rather than the book itself. This is, of course, exactly what we have. The actual book has been withheld. Only words from the book have been given us. But those words are intended to be of great worth to mankind, and in particular to the remnant.
This process is sacred, the promises are from the Lord. These words are given to us by Him, through a servant possessing authority to seal them up. We cannot prevent them from happening. We can, however, align ourselves with them and in turn be saved as well.