JOEL - These things were made known to man solely by revelation from God through the prophets of the Book of Mormon.
Probably the single most important reason for the coming forth of the Book of Mormon is for the "convincing of the Jew and Gentile that JESUS is the CHRIST, the ETERNAL GOD," (Title Page). This was the main objective on the prophet Mormon's mind as he compiled the Book from the hundreds of records that had been handed down from past prophets.This is evident in the following verses:

4 "And the things which are upon these plates pleasing me, because of the prophecies of the coming of Christ; and my fathers knowing that many of them have been fulfilled; yea, and I also know that as many things as have been prophesied concerning us down to this day have been fulfilled, and as many as go beyond this day must surely come to pass--
5 Wherefore, I chose these things, to finish my record upon them, which remainder of my record I shall take from the plates of Nephi; and I cannot write the hundredth part of the things of my people.
6 But behold, I shall take these plates, which contain these prophesyings and revelations, and put them with the remainder of my record, for they are choice unto me; and I know they will be choice unto my brethren."(Words Of Mormon 1:4-6)

As Mormon studied all the records, he specifically gleaned out anything that had to do with Jesus Christ, because he knew they would "be choice unto my brethren". This is one of the reasons that there are so many references and revelations about Christ throughout the book. Also, the books of 1 Nephi through Omni were written down by Mormon exactly as they appeared on the "small plates of Nephi". But the books, Words of Mormon through Mormon, are written mostly in Mormon's own words, as he pulled information from the "large plates of Nephi"; often providing some comentary as he deemed appropriate. His personal knowledge and awareness of the mission of Christ, which to him was now recorded history, probably had an influence on his choice of material and comentary as he recorded the historical events and the prophesies concerning Christ.

Sometimes while reading the Book of Mormon people have a hard time understanding how such specific and detailed prophesies can be made about Jesus Christ, compared to the rather vague prophesies made by prophets thoroughout the Old Testament. The main reason has to do with the translation history of both records. The Book of Mormon was translated relatively recently, only once, by only one prophet, by the power and inspiration of God. The Bible has undergone many translations, over centuries of time, by many so-called Biblical scholars and theologians, who may or may not have been inspired by God; their translations perhaps reflecting their own biases and personal beliefs (See The History of the English Bible). There is also the possibility that through the centuries, prophesies and doctrines, relating to the coming of Christ, may have been lost.