According to Robert Parsons, if we accept this meaning of the phrase, "the book proceeded forth from the mouth of a Jew" (1 Nephi 13:24), we would have to accept the idea that Ezra's collection of writings more nearly approached what was in the brass plates than the writings now contained in our Old Testament, since the angel also told Nephi that the book contained the fulness of the gospel. The fulness of the gospel of necessity would include a correct understanding of the creation, the fall, the atonement, and the principles of faith, repentance, baptism, and the Holy Ghost. All of these are essential for one to enter the celestial kingdom, and the fulness of the gospel prepares and qualifies one to enter that kingdom. Since the Pearl of Great Price and the Joseph Smith Translation of the Bible make it abundantly clear that much has been lost from the Old Testament, and since our King James Version of the Bible agrees to a great extent with the Dead Sea Scrolls, it seems apparent that many Old Testament losses occurred before the time of Christ. However, since Paul seems familiar with the Tame and Wild Olive Tree allegory (Romans 11:17), and since Jude refers to writings in the book of Enoch (Jude 1:14), perhaps losses after the time of Christ also occurred. Elder Mark E. Petersen wrote: "When Nephi spoke of the 'plain and precious' parts of the scripture which were eliminated he spoke of the witness of Christ which is no longer in the Old Testament."
In light of all we know today, perhaps a better interpretation of "the book [that] proceeded forth from the mouth of a Jew" would be to identify it as the Bible, which came from the mouth of Jesus and also the mouths of many Jews. This would be consistent with 1 Nephi 13:24, which states that the book contained the fulness of the gospel of the Lord, of whom the twelve apostles bore record, and verse 25, which states that "these things [the book] go forth from the Jews in purity unto the Gentiles." However, a distinction should probably be made between the Old Testament and the New Testament. The Greek Septuagint translation (around 250 B.C.) and the Dead Sea Scroll Old Testament manuscripts (around 200 B.C. to A.D. 70) were already corrupted before the New Testament was written. The New Testament was pure at first but was later corrupted. Thus, there never was a time in which both the Old Testament and the New Testament were uncorrupted at the same time, though the component parts of each of the testaments were pure when they "came from the pen of the original writers." [Robert E. Parsons, "The Great and Abominable Church," in Studies in Scripture: Book of Mormon, Part 1, pp. 48-51]