The Book of Mormon is a volume of holy scripture comparable to the Bible and is a record of God's dealings with His chosen people in the New World. The main purpose of the Book of Mormon is "to the convincing of Jew and Gentile that JESUS is the CHRIST, the ETERNAL GOD, manifesting himself unto all nations." (Book of Mormon Title Page) It was written by ancient American prophets for our day (Mormon 8:35) and is an American testament of the Lord Jesus Christ.
Sometimes people make the mistake of thinking Mormons believe the Book of Mormon teaches everything there is to know about Jesus. I'm not sure how this myth got started, but it's certainly not something taught in LDS Sunday School or any other curriculum of the Church. Therefore, there's no reason for anyone outside of the Church to believe it either.
Ironically, the Bible is also often held up against the Book of Mormon as a complete and perfect standard of all knowledge about everything...everything...about Jesus, even to the point that nothing more can or should be written.
But that belief contradicts John 21:25, which says:
And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen.
The simple mathematical fact is that no one book can contain everything a person said or did. Any biographer can tell you this with 100% certainty. It's simply impossible for mortal ears and eyes to convert to writing the sheer volume of all events and speech in even one person's life. We were not meant to look at any work of scripture, Bible or Book of Mormon, as a universal compendium of everything there is to know about a person or events or even doctrine!
Rather, the highlights and key events and doctrines in the lives and speech of the prophets and of Jesus Christ are recorded in these books as a way to unify truth into one great whole.
More knowledge will be given to us later, if we can show that we're willing to believe and obey what we've already been given.
In the Book of Mormon, the prophet Mormon included a statement by Nephi's younger brother, Jacob, about the fact that not everything the people of Nephi did could be recorded in one book, or even in many books.
And a hundredth part of the proceedings of this people, which now began to be numerous, cannot be written upon these plates; but many of their proceedings are written upon the larger plates, and their wars, and their contentions, and the reigns of their kings.
Indeed, both the Bible and the Book of Mormon affirm that there are other nations that had been touched by Gospel teachings. Those experiences were recorded.
Neither at any time hath the Father given me commandment that I should tell unto them concerning the other tribes of the house of Israel, whom the Father hath led away out of the land.
This much did the Father command me, that I should tell unto them:
That other sheep I have which are not of this fold; them also I must bring, and they shall hear my voice; and there shall be one fold, and one shepherd.
The following instances in the Book of Mormon show where more is mentioned or said that was not actually written, or that was or is yet to be written, or that was written and was subsequently lost (the 116 translated pages that Martin Harris lost) or not included (material that Mormon simply summarized or just didn't include in his abridgment), or that was written and we just don't know about it yet.
And now I, Nephi, do not make a full account of the things which my father hath written, for he hath written many things which he saw in visions and in dreams; and he also hath written many things which he prophesied and spake unto his children, of which I shall not make a full account.
But I shall make an account of my proceedings in my days. Behold, I make an abridgment of the record of my father, upon plates which I have made with mine own hands; wherefore, after I have abridged the record of my father then will I make an account of mine own life.
And it mattereth not to me that I am particular to give a full account of all the things of my father, for they cannot be written upon these plates, for I desire the room that I may write of the things of God.
And after this manner of language did my father prophesy and speak unto my brethren, and also many more things which I do not write in this book; for I have written as many of them as were expedient for me in mine other book.
And behold, the things which this apostle of the Lamb shall write are many things which thou hast seen; and behold, the remainder shalt thou see.
But the things which thou shalt see hereafter thou shalt not write; for the Lord God hath ordained the apostle of the Lamb of God that he should write them.
And also others who have been, to them hath he shown all things, and they have written them; and they are sealed up to come forth in their purity, according to the truth which is in the Lamb, in the own due time of the Lord, unto the house of Israel.
And I, Nephi, heard and bear record, that the name of the apostle of the Lamb was John, according to the word of the angel.
And behold, I, Nephi, am forbidden that I should write the remainder of the things which I saw and heard; wherefore the things which I have written sufficeth me; and I have written but a small part of the things which I saw.
For I, Nephi, was constrained to speak unto them, according to his word; for I had spoken many things unto them, and also my father, before his death; many of which sayings are written upon mine other plates; for a more history part are written upon mine other plates.
Wherefore, because that ye have a Bible ye need not suppose that it contains all my words; neither need ye suppose that I have not caused more to be written.
For I command all men, both in the east and in the west, and in the north, and in the south, and in the islands of the sea, that they shall write the words which I speak unto them; for out of the books which shall be written I will judge the world, every man according to their works, according to that which is written.
And as these plates are small, and as these things are written for the intent of the benefit of our brethren the Lamanites, wherefore, it must needs be that I write a little; but I shall not write the things of my prophesying, nor of my revelations. For what could I write more than my fathers have written? For have not they revealed the plan of salvation? I say unto you, Yea; and this sufficeth me.
And then shall ye know that I have seen Jesus, and that he hath talked with me face to face, and that he told me in plain humility, even as a man telleth another in mine own language, concerning these things;
And only a few have I written, because of my weakness in writing.
And he did minister many things unto them; and all of them cannot be written, and a part of them would not suffice, therefore they are not written in this book. And Nephi did minister with power and with great authority.
Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord.
Wherefore, I write a few more things, contrary to that which I had supposed; for I had supposed not to have written any more; but I write a few more things, that perhaps they may be of worth unto my brethren, the Lamanites, in some future day, according to the will of the Lord.