Before 1981, the Book of Mormon was simply the Book of Mormon; since then, however, it has borne the subtitle “Another Testament of Jesus Christ.” I suspect that this addition had several purposes. First almost certainly was to draw attention to Christ in a church that many consider non-Christian. Second would be to clarify the relationship of the book to the Bible: not a replacement, but An Other Testament like the Old and New ones.
Growing up, I always read “testament” as “testimony”: in Mormonsprach, a declaration of faith in Jesus Christ or some aspect of His Gospel. Just as I or any other Mormon might testify of Christ’s atoning power, the prophets in writing scripture did the same thing. This, I believe, is how most Mormons read the Book of Mormon subtitle and the subdivisions of the Christian Bible: the Old Testimony, the New Testimony, and the Other Testimony.
Looking at translations clarifies that “Another Testament of Jesus Christ” was chosen due to the supposed testament/testimony synonymy. In languages without a Romance cognate of “testament” we see that “testimony” is, indeed, the desired meaning: in Arabic we have شهادة (shahada), the same word as the declaration of faith that makes one Muslim, and in Greek we have μαρτυρία, borrowed into English as martyr—both of which mean “witness.”
However, as I became better versed in the Bible (especially non-KJV and non-English versions), I began to notice something awkward: “testament” is not, after all, equivalent to “testimony.” In fact, when the word appears in the NIV’s telling of the Last Supper (KJV: “this is my blood of the new testament”) it is translated as “covenant”; the Spanish Reina-Valera uses “pacto” (meaning “pact” or “covenant”). Some Protestant denominations even break free of traditional scriptural nomenclature and call the Biblical subdivisions the “New Covenant” and the “Old Covenant.”
This better understanding of the Biblical use of “testament” keyed me into why some non-Mormon Christians might be disturbed, not comforted, by the invocation of Christ in the Book of Mormon’s subtitle. The use of “testament” terminology—particularly in the epistles—specifically contrasts the “old” Mosaic covenant of sacrifices and performances with the “new” Christian covenant of grace; the Law of Moses is fulfilled and superseded by the Law of the Gospel. If the Book of Mormon’s claim to be “Another Testament of Jesus Christ” is read in this light, it seems to promise a God-given law superseding Christ’s atonement! This is an obvious problem—and one that I am unsure how to address.
Does the Book of Mormon, however, present “Another Covenant of Jesus Christ”? If so, what is that covenant?
Some latter-day revelations might give us hints on this matter. The famous scripture Ezra Taft Benson used to kickstart serious study of the Book of Mormon, for example, was Doctrine and Covenant 84:57, in which the Lord says to Joseph Smith that the whole church “shall remain under this condemnation [brought about by “vanity and unbelief,” v. 55] until they repent and remember the new covenant, even the Book of Mormon and the former commandments which I have given them, not only to say, but to do according to that which I have written.” How can the Book of Mormon itself be a covenant?
Another of the D&C passages to refer to the Book of Mormon, 20:8-12, notes that the Book of Mormon “was given by inspiration, and is confirmed to others by the ministering of angels, and is declared unto the world by them— Proving to the world that the holy scriptures are true, and that God does inspire men and call them to his holy work in this age and generation, as well as in generations of old” (D&C 20:10-11). In these verses, it seems like the very existence of the Book of Mormon obligates the believer to certain corollaries: angels minister, God still inspires and calls men, the holy scriptures are true. This could be relevant to talk of a “Book of Mormon covenant,” a covenant to believe in God and His prophets, past and present, Jewish, Christian, and Mormon.
This would fit with the Mormon reception history of the Book of Mormon: namely, it has most often been used as proof of Joseph Smith’s prophetic calling instead of a source of doctrine. However, there are several strands of thought in the Book of Mormon (beyond the fact that it constantly decries the heresy of cessationism—the belief that miracles, revelation, and scripture have ceased) that gives weight to this idea that its covenant dictates a belief in continued divine inspiration.
Chronologically, the Book of Mormon starts out in a Judaic setting with mysterious visionary prefigurations of Christianity. After the Lehites leave Jerusalem, the “Messiah” becomes increasingly identified with a specific historical figure yet to come, and by the time Nephi’s brother Jacob is preaching, he begins using the title “Christ” to speak of this future savior. Nevertheless, the righteous peoples strive to keep the Law of Moses, living a sort of bizarrely Christian-inflected Judaism. When one man, Sherem, questions Jacob’s focus on this Christ by arguing that he is adulterating the Law of Moses, he is smitten by God, and Jacob’s Judeo-Christian ambiguity carries the day. When some after the sign of Christ’s birth argue that the Law of Moses should be done away with, they are convinced otherwise by those who assert that “the law was not yet fulfilled.” When Christ, having completed His sacrifice, appears to them, only then does he give them permission to move ahead from the Mosaic Law. Thus, in the Book of Mormon the righteous actors were very attentive to how God had instructed them to live, practicing rituals with full knowledge that someday, at the appropriate time, they will be “fulfilled”; that God had not ceased speaking, and could still add to His truth revealed. Furthermore, in the Book of Mormon Christian ecclesiastical universalism does not displace earlierIsraelite covenants at all; Mormons are not supercessionists. Some of the older covenants—including, for instance, that the non-Gentile (i.e., native) inhabitants of the Americas will eventually rule those continents again—have yet to be fulfilled.
While this may not be what the authors of the subtitle had in mind, this dedication to God’s received and continuing revelations is implicit and necessary if one accepts the Book of Mormon, no matter whether you regard it as a sign or a narrative. To declare in unbelief that God has ceased speaking or calling people to preach on His behalf would be breaking that nigh-covenantal dedication.
So while the Old Covenant prefigures Christ and the New Covenant presents Him, the Other Covenant can bridge the two, providing a model for faithful liminality then and in our day.
 Ironically, the Book of Mormon in its doctrine cleaves pretty close to the New Testament, exhibiting few or none of the distinctive Mormon beliefs, such as temple worship, tiered heavens, and divinization, that people see as typically Mormon heresies.
 Unfortunately, we are left uncertain as to the identity of the “former commandments”: are these Joseph Smith’s revelations, or the Bible? As it is addressed to the LDS church at the time, I’m leaning toward the former.
 On a basic level, it seems like D&C 84:57 is an injunction against hypocrisy: the scriptures put their readers under condemnation if those readers profess belief but do not act accordingly. That seems a little too general.
 2 Nephi 29 has some strong words to those who would say otherwise.