Mosiah 1:3; “And he also taught them concerning the records which were engraven on the plates of brass, saying: My sons, I would that ye should remember that were it not for these plates, which contain these records and these commandments, we must have suffered in ignorance, even at this present time, not knowing the mysteries of God.”
Romans 16:25-26; “Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith.”
According to the Book of Mormon, the mysteries of God could only known through the plates of brass Nephi stole after he murdered Laban. These plates supposedly contained the Torah, and writings from some of the OT prophets – mainly Isaiah and Malachi. In Mosiah 1:5 he actually said if God had told them earlier they wouldn’t have ‘dwindled in unbelief’.
Mosiah reiterated his message a little later and stated –
Mosiah 2:9; “…open your ears that ye may hear, and your hearts that ye may understand, and your minds that the mysteries of God may be unfolded to your view.”
Keep in mind Mosiah lived circa 124 BC…
But the BoM gives even more ‘insight’ when their prophet Alma also expounded on the mysteries of God. This time it was about 40 years later, circa 82 BC.
Alma 12:30, 32; “And they began from that time forth to call on his name; therefore God conversed with men, and made known unto them the plan of redemption…32 Therefore God gave unto them commandments, after having made known unto them the plan of redemption, that they should not do evil, the penalty thereof being a second death, which was an everlasting death as to things pertaining unto righteousness; for on such the plan of redemption could have no power, for the works of justice could not be destroyed, according to the supreme goodness of God.”
As the Apostle Paul stated, God hadn’t fully revealed His plan of redemption until the first century, which is when the gospel was presented to mankind! How is it Mosiah and company heard about this more than a century before Jesus was born? Also see Ephesians 3:3-5 –
“How that by revelation he made known unto me the mystery; (as I wrote afore in few words, 4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit.”