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). Likewise, in ancient religions, for example from the Hellenistic world, the word mysteries was often used to describe "cultic rites . . . portrayed before a circle of devotees," who "must undergo initiation" and who are promised "salvation by the dispensing of cosmic life," which is sometimes "enacted in cultic drama," accompanied by a strict "vow of silence." (John W. Welch, "The Temple in the Book of Mormon," in Temples of the Ancient World, pp. 364-366] [See the commentary on Alma 12:9]
“Having Had a Great Knowledge of the Goodness and the Mysteries of God”
Avraham Gileadi notes that in the Interpreter's Dictionary of the Bible (3:352-53), the term "goodness" is listed as a synonym of covenant blessing and covenant keeping. He writes:
Like Lehi, his exemplary father, Nephi comes to know personally the "goodness" and mysteries of God (1 Nephi 1:1). Even at a young age, Nephi becomes a prophet in his own right--he sees the Lord in a vision, thus making sure his calling and election (1 Nephi 11:7, 21, 27; 18:3). . . .
In another part of the Book of Mormon account, we find a circumstance that might shed some additional light on the term "goodness." Under conditions of widespread wickedness, Mormon, at age fifteen, becomes chief captain of the Nephite armies (Mormon 2:1-2). This is the same Mormon after whom the Book of Mormon is named. More to the point, however, Mormon writes the following: "And I, being fifteen years of age and being somewhat of a sober mind, therefore I was visited of the Lord, and tasted and knew of the goodness of Jesus." (Mormon 1:15) . . .
The parallelism of these two statements implies that to know the goodness of Jesus is to be visited of the Lord, to make one's calling and election sure. [Abraham Gileadi, The Last Days: Types and Shadows from the Bible and the Book of Mormon, pp. 216, 218, 232] [See the commentary on Mormon 1:15]
“The Mysteries of God”
Critics of the Book of Mormon claim that the words "the mysteries of God" in 1 Nephi 1:1 were taken from 1 Corinthians 4:1. In parallel columns of similar passages from the Book of Mormon and from the New Testament, they attempt to show how Joseph Smith "plagiarized" the New Testament.
According to John Tvedtnes, what concerns him most is that these critics neglect to tell their readers that many of the Book of Mormon concepts and phrases which they claim were borrowed from the New Testament are also found in the Old Testament. Some of them are merely common phrases found in Jewish culture, and in some cases, the New Testament is actually quoting from the Old Testament. Or, as in this case, the word appears only in the New Testament because different parts of the King James Bible were translated by different committees, and the Old Testament translators chose to use a different word or phrase. As for the word "mysteries," the Old Testament translators chose to use the word "secret." The term "secret of God" appears in Job 15:8; 29:4.
Tvedtnes notes that the Book of Mormon critics also argue that the Book of Mormon should not use the same language as the Bible, since it was translated from a different tongue. They particularly object to the fact that the Book of Mormon, when using biblical passages, employs the form found in the King James Bible.
Tvedtnes' response to this criticism is that Joseph Smith deliberately used the King James Version wording because it corresponded to the Bible known to his contemporaries. (See Hugh W. Nibley, "Literary Style Used in Book of Mormon Insured Accurate Translation," in The Prophetic Book of Mormon, vol. 8, 212-18.) His work would undoubtedly not have been well-received had he done otherwise. The use of precise New Testament phraseology is not negative as long as the idea fits the passage. After all, Joseph Smith rendered the Book of Mormon in English theological terms of his day, most of which derived from the King James Bible. Critics who propose such plagiarism must explain how Joseph Smith could be so brilliant to compose such a seamless profusion of biblical scripture in the Book of Mormon but stupid enough to believe that he could get away with using New Testament quotes from pre-Christian times. [John A. Tvedtnes, Book Review of Jerald and Sandra Tanner's Covering Up the Black Hole in the Book of Mormon, in Review of Books on the Book of Mormon, Vol. 3 1991, pp. 217-219] [See the commentary on 1 Nephi 11:6-7; 2 Nephi 12-24]