The importance of the historicity of the Book of Mormon was recognized at the beginning of its production. Even before it was sold to the general public, a mocking thief published what was purported to be part of its pages. It read like what was expected from a farm boy with little education, complete with typical Indian representations that last even today. When the book came out, the complexity and seriousness of it forced a stronger attack. Joseph Smith could no more be the author, but a far more educated individual had to have been responsible. When this didn't seem to cut it (although such arguments still exist) then it was the product of Joseph Smith's environment he got from every and any corner his curiosity could find. This remains the dominant criticism of the book as historical with DNA the supposed nail in the coffin. This ignores that the book is as spiritually and literarily forceful today and reads far more like modern(ist) literature than the romanticism of its publishing era. The authors are self aware, holy scripture is considered amendable and reductionist, editorial comment often contains meta-narrative, and the writing process itself is explored. The depth is absolutely astounding.
Over at Times and Seasons, Steve Smith asks the question, "So why must a scholar of Mormon Studies feel compelled to take a stance with regard to many issues such as Book of Mormon historicity?" He likens it to "bracketing" the Garden of Eden, The Flood, and so forth. This brazen question is astonishing for anyone who is familiar with the Book of Mormon's own text and arguments. Joseph Smith called it, "the most correct of any book on earth, and the keystone of our religion, and a man would get nearer to God by abiding by its precepts, than by any other book." He spent a lifetime reiterating and gathering evidence for the historical truth of the Book of Mormon, and not just its teachings. His short commentary on a book about Central American ruins is the most famous example. There was only one other subject that Joseph Smith declared a foundation for Mormonism, and that the Divinity of Jesus Christ. He stated, "The fundamental principles of our religion are the testimony of the Apostles and Prophets, concerning Jesus Christ, that He died, was buried, and rose again the third day, and ascended into heaven; and all other things which pertain to our religion are only appendages to it." There was no equivocation of the reality of Jesus as Savior and there shouldn't be for the historical truth of the Book of Mormon if you take Joseph Smith seriously.
The History outside of Historicity
Those who reject the historicity of the Book of Mormon often quickly dismiss the events related to its writing. From the start there was a physical element to the spiritual that transcended the text. Joseph Smith came home and told his family the encounters with an Angel that had a mortal life in the pages of the book. This Angel, according to his mother as told by him, would tell Joseph Smith the history of the Nephites or ancestors of the Native Americans. This same Angel told him where to find the gold plates on the very real land near the family farm.
He was not alone in having an encounter with the golden book or the people in its narrative. His wife often felt the plates from under a sheet. It is even hinted at in Doctrine and Covenants 8 and 9 that Oliver Cowdery handled the gold plates and seers in the process of trying to translate them by the Gift of God. Of course, there is the famous eight witnesses who handled the plates and the three that witnessed both the plates and the Angel Moroni. The fact these plates were taken back to heaven is galling to some, but Joseph Smith wasn't the only one to claim to see or handle them despite popular depictions of his lone testimony.
There were, more importantly, practical theological developments based on the idea of the historicity of the book. The Priesthood is among the most directly inspired by believing in the narrative:
We still continued the work of translation, when, in the ensuing month (May, 1829), we on a certain day went into the woods to pray and inquire of the Lord respecting baptism for the remission of sins, that we found mentioned in the translation of the plates. While we were thus employed, praying and calling upon the Lord, a messenger from heaven descended in a cloud of light, and having laid his hands upon us, he ordained us, saying: Upon you my fellow servants, in the name of Messiah, I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth until the sons of Levi do offer again an offering unto the Lord in righteousness. . . . The messenger who visited us on this occasion and conferred this Priesthood upon us, said that his name was John, the same that is called John the Baptist in the New Testament, and that he acted under the direction of Peter, James and John, who held the keys of the Priesthood of Melchizedek, which Priesthood, he said, would in due time be conferred on us, and that I should be called the first Elder of the Church, and he (Oliver Cowdery) the second. It was on the fifteenth day of May, 1829, that we were ordained under the hand of this messenger, and baptized.
From the supposed non-historical Book of Mormon came a very historical claimed event. Jumping from non-reality to reality so quickly and easily must bring the whole into question. This is repeated time and again. A bit of evidence includes the Anthon Transcript incident that is not rejected as historical, and yet it wouldn't have happened if not for the Book of Mormon's interpretation of Isaiah to prove itself. Narrative flows into history that then goes back upon itself to the text.
What the writers in the Book of Mormon say about the historicity of the text is equally as precise in its claims. "I Nephi, having been born of goodly parents," (1 Nephi 1:1) is a statement of a life having been lived. Each prophet and writer mentioned in the text somehow relates back to these first words. There is a long chain of generations that stretches until the last prophet Moroni, "Behold, I make an end of speaking concerning this people. I am the son of Mormon, and my father was a descendant of Nephi," where he ties his identity in with the gold plates that Joseph Smith will interpret. "And I am the same who hideth up this record unto the Lord; the plates thereof are of no worth, because of the commandment of the Lord. For he truly saith that no one shall have them to get gain; but the record thereof is of great worth; and whoso shall bring it to light, him will the Lord bless." (Moroni 8:13-14).
Like the Bible, lineage is of great importance to the identity of the individual's history. Who one is born from has implications for responsibilities and expectations. "I am Ammoron, and a descendant of Zoram," (Alma 54:23), "He that wrote this record was Ether, and he was a descendant of Coriantor."(Ether 1:6), "Now, among those who joined the people of the Lord, there were none who were Amalekites or Amulonites, or who were of the order of Nehor, but they were actual descendants of Laman and Lemuel."(Alma 24:29). To make the case that these were not merely made up bloodlines, Nephi and others made sure to indicate they came from an even more distinguished background. They came from Joseph who was a slave in Egypt before becoming its protector, "And thus my father, Lehi, did discover the genealogy of his fathers. And Laban also was a descendant of Joseph, wherefore he and his fathers had kept the records."(1 Nephi 5:16). Records upon records, with family relations stretching back to a Jewish hero testify for real people contained in its pages.
This isn't all, but the words are testified to in no uncertain terms. Nephi ends his writing by declaring (2 Nephi 33:10-11):
10 And now, my beloved brethren, and also Jew, and all ye ends of the earth, hearken unto these words and believe in Christ; and if ye believe not in these words believe in Christ. And if ye shall believe in Christ ye will believe in these words, for they are the words of Christ, and he hath given them unto me; and they teach all men that they should do good. 11 And if they are not the words of Christ, judge ye—for Christ will show unto you, with power and great glory, that they are his words, at the last day; and you and I shall stand face to face before his bar; and ye shall know that I have been commanded of him to write these things, notwithstanding my weakness.
After the record of Jesus Christ visiting the lands of Nephi, we read (3 Nephi 29:1-9):
1 And now behold, I say unto you that when the Lord shall see fit, in his wisdom, that these sayings shall come unto the Gentiles according to his word, then ye may know that the covenant which the Father hath made with the children of Israel, concerning their restoration to the lands of their inheritance, is already beginning to be fulfilled. 2 And ye may know that the words of the Lord, which have been spoken by the holy prophets, shall all be fulfilled; and ye need not say that the Lord delays his coming unto the children of Israel. 3 And ye need not imagine in your hearts that the words which have been spoken are vain, for behold, the Lord will remember his covenant which he hath made unto his people of the house of Israel. 4 And when ye shall see these sayings coming forth among you, then ye need not any longer spurn at the doings of the Lord, for the sword of his justice is in his right hand; and behold, at that day, if ye shall spurn at his doings he will cause that it shall soon overtake you. 5 Wo unto him that spurneth at the doings of the Lord; yea, wo unto him that shall deny the Christ and his works! 6 Yea, wo unto him that shall deny the revelations of the Lord, and that shall say the Lord no longer worketh by revelation, or by prophecy, or by gifts, or by tongues, or by healings, or by the power of the Holy Ghost!
Moroni is no less adamant that he exists as a person and the records are in fact real. He leaves with a warning for those who judge them harshly, and personal reasons for writing (Mormon 9:30-35):
30 Behold, I speak unto you as though I spake from the dead; for I know that ye shall have my words. 31 Condemn me not because of mine imperfection, neither my father, because of his imperfection, neither them who have written before him; but rather give thanks unto God that he hath made manifest unto you our imperfections, that ye may learn to be more wise than we have been. 32 And now, behold, we have written this record according to our knowledge, in the characters which are called among us the reformed Egyptian, being handed down and altered by us, according to our manner of speech. 33 And if our plates had been sufficiently large we should have written in Hebrew; but the Hebrew hath been altered by us also; and if we could have written in Hebrew, behold, ye would have had no imperfection in our record. 34 But the Lord knoweth the things which we have written, and also that none other people knoweth our language; and because that none other people knoweth our language, therefore he hath prepared means for the interpretation thereof. 35 And these things are written that we may rid our garments of the blood of our brethren, who have dwindled in unbelief.
There is a strong sense of historicity running through the text, with no hint that what is narrated shouldn't be taken as facts. The opposite has to be said. Every sentence is written in a way that screams to be taken at face value. It wants to be accepted or rejected with no half measures. As Ben S. explains about the title page in the Times and Season discussion on apologia:
1) “show[ing] unto the remnant of the house of Israel what great things the Lord hath done for their fathers.” Now, if there are no Nephites, then God didn’t actually do anything for the remnant of the house of Israel, as described therein. 2) “that they may know the covenants of the Lord, that they are not cast off forever.” Those covenants are described, remade, and modeled several times in the Book of Mormon. But again, if there were no Nephites at all, this is nonsensical, and God never did so. 3) “convincing of the Jew and Gentile that Jesus is the Christ, the Eternal God, manifesting himself unto all nations”. But again, if there were no Nephites and no Christ-visit to them, than God has not manifested himself to all nations. So the Book of Mormon is nonsensical as a divinely-inspired allegory, because the genre of allegory cannot sustain the historical goals it sets for itself in witnessing to these things. If you bring in a legal witness who says “yes, so and so actually came to my house, was present. Well ok, that never actually happened at all, I’m just making an allegory about his character”, it’s not a valid witness.
There are some very specific claims made inside the book. Miracles are real! Jesus is the Christ! The Nephites and Lamanites did live and die to testify of God's promises and warnings! The Book of Mormon can only be understood and believed in the context of the history its narrative presents and interacts with on each page.
The Bible is the Guide
As pointed out, there is a deep connection between the persons and events of the Bible and what is in the Book of Mormon. Once again, we turn to 1 Nephi 1:4 where a very historical connection to the Book of Mormon narrative is set down, "For it came to pass in the commencement of the first year of the reign of Zedekiah, king of Judah, (my father, Lehi, having dwelt at Jerusalem in all his days); and in that same year there came many prophets, prophesying unto the people that they must repent, or the great city Jerusalem must be destroyed." Here we have a name Zedekiah, a city Jerusalem, and prophesied event the destruction of Jerusalem that is historical. Even the time period of 600 B.C. can be pointed out. The implications are clear even if a few try to ignore it; we are dealing with a claim of facts.
The last pages teach how to gain a testimony of the Book of Mormon. Its focus in Mormoni 10:3 is on the historical blessings of God, "Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts." More than words and teachings are at the heart of a testimony, but history itself. The Bible is the center of our pondering. We must consider what God has done throughout the human narrative, and the Book of Mormon events are part of that tapestry.
All of this is a preliminary discussion into the ways believing in the historicity of the Book of Mormon is essential. For the same reason it wouldn’t be such a good idea to deny that Jesus Christ really is the Son of God and Savior of the World who was Resurrected and Atoned for our sins. Sure, you could be a good moral Mormon in name, but it would be rejecting a core theology. Mormonism as a religion would be made impotent. For most faithful and active Mormons, the historicity of the Book of Mormon is as much a cornerstone of the faith as Jesus Christ.