"And Alma spake many more words unto the people, which are not written in this book."
This is often the case. John's Gospel ended with this observation:
Mormon was the editor of this portion of the Book of Mormon. He was the one who determined to omit portions what Alma said to these people on this occasion. From what Mormon left for us to read, his intent is clear. He wanted us to understand the bigger picture of God's dealings with man, man's possession of priestly power, and the importance of repentance and defeating religious error. Mormon had seen us, and included specific warnings addressed to us, the Gentiles. He cautioned us about the Book of Mormon as follows:
"And then, O ye Gentiles, how can ye stand before the power of God, except ye shall repent and turn from your evil ways? Know ye not that ye are in the hands of God? Know ye not that he hath all power, and at his great command the earth shall be rolled together as a scroll? Therefore, repent ye, and humble yourselves before him, lest he shall come out in justice against you—lest a remnant of the seed of Jacob shall go forth among you as a lion, and tear you in pieces, and there is none to deliver." (Mormon 5: 22-24.)
Mormon knew the book would initially be in the hands of the Gentiles. So you can know we are identified as "Gentiles" in the Book of Mormon. Also, Joseph Smith declared in the dedicatory prayer for the Kirtland Temple that we are identified with the Gentiles. (D&C 109: 59-60.) Although Brigham Young and President Joseph Fielding Smith taught that Joseph was a "pure blooded Ephramite." (Doc. Sal. Vol 3: 253-54.)
The selected materials that Mormon gave to us were targeted to the purpose of the Book of Mormon. The title page (written by Moroni) tells us the purpose: "Written to the Lamanites, who are a remnant of the house of Israel; and also to Jew and Gentile—Written by way of commandment, and also by the spirit of prophecy and of revelation—Written and sealed up, and hid up unto the Lord, that they might not be destroyed—To come forth by the gift and power of God unto the interpretation thereof—Sealed by the hand of Moroni, and hid up unto the Lord, to come forth in due time by way of the Gentile."
Gentiles would receive, translate and disseminate the Book of Mormon. But the Gentiles are prophesied to fail in their faith. They are to become full of their own abominations. When they reject the fullness of the Gospel, then it will be taken from them and given back to the remnant. (3 Ne. 16: 10-11.)
Mormon is using the message from Alma to provide to the Gentiles (who will reject the invitation), an opportunity to understand the fullness which was offered to them. It was intended to remove from them the excuse that they were not given an opportunity and did not understand. Therefore, the Book of Mormon's primary purpose, to make the Gentiles aware and accountable for their failure, is accomplished by Mormon including this portion of Alma's teachings. The fact that other portions were left out mean that they would not have contributed to the task before Mormon.
A few of the Gentiles happily may be numbered with the remnant. (3 Ne. 16: 13.) That is conditioned upon their repentance. The degree and completion of that repentance is shown by this portion of the sermon by Alma which Mormon preserved for us.
We are on notice. We are accountable for how we react to that notice. For the most part, the expectation is that we tell one another in reassuring words that "all is well." and that "Zion prospers," and to generally allow our souls to be cheated while we are led carefully down to hell. (2 Ne. 28: 21-25.) Still, some few will follow Christ, despite the leaders' teachings that will cause them to err. (2 Ne. 28: 14.)
The Book of Mormon is a record that will be used as evidence we have been warned. In plain language and with sufficient truth to hold us all accountable, this is the standard by which we are to find our way back to the Lord in this last dispensation before His return. We remain, of course, under condemnation because we are unwilling to do that. (D&C 84: 57.)
What a great and terrible book. What an alarming message. It is no wonder we neglect it so.