Upon his arrival at the land of Lehi-Nephi, Ammon was brought to meet with king Limhi. Among the things which king Limhi brought up in their recorded conversation was the existence and possible translation of the 24 plates which were brought back by an expedition of men. King Limhi pointedly asks Ammon: "Canst thou translate?" . . . Knowest thou of any one that can translate?" (Mosiah 8:11-12). Ammon then launches into an elaborate explanation about the king of Zarahemla having "interpreters" and being a "seer" (Mosiah 8:13-18). At the end of this explanation, king Limhi "gave thanks to God, saying: Doubtless a great mystery is contained within these plates, and these interpreters were doubtless prepared for the purpose of unfolding all such mysteries to the children of men.
One might ask, What is a "mystery"? And why would king Limhi suppose a "great mystery" was contained within the plates? According to John Welch, the word mysteries seems to refer to priesthood [covenants] or temple ordinances. Benjamin unfolded the "mysteries of God" to his people by speaking to them at the temple (Mosiah 2:9). [John W. Welch, "The Temple in the Book of Mormon," in Temples of the Ancient World, pp. 364-366] [See the commentary on 1 Nephi 1:1; Alma 12:9]
According to Kevin and Shauna Christensen, although many explanations could be made for the possible meaning of the "great mystery," the only other reference to "a great mystery" in all the scriptures comes in Paul's discussion of the love husbands should have for their wives when the two are joined into one flesh (Ephesians 5:28-32). That is, the "great mystery" has to do with marriage [or covenant marriage], for the term mystery should not be understood as something generally unknown, but refers specifically to the temple mysteries. Such a context for the specific notion of "a great mystery" implies a hunger for greater knowledge of the temple [covenant] ordinances and mysteries. [Kevin and Shauna Christensen, "Nephite Feminism Revisited: Thoughts on Carol Lynn Pearson's View of Women in the Book of Mormon," in FARMS Review of Books, Volume 10, Number 2, 1998, pp. 30-31]