In an article written by Sandra Tanner, "Bible and Book of Mormon Contradictions," we read the following purported "contradiction" between the two volumes:
The Bible plainly states that the gospel, with its inclusion of Gentiles, was not fully revealed until after Christ's death.
1. Eph. 3:3-7 Paul writes "by revelation he made known unto me the mystery; (as I wrote afore in few words, whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 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; that the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: whereof I was made a minister ...... (See also Col. 1:26; 1 Peter 1: 1-12; Romans 16:25-26)
However, the Book of Mormon maintains that this knowledge was had in 545 B.C.
2 Nephi 25:19 "For according to the words of the prophets, the Messiah cometh... his name shall be Jesus Christ, the Son of God. ... (v.23) For we labor... to persuade... our brethren, to believe in Christ, and to be reconciled to God; for we know that it is by grace that we are saved, after all we can do.
2 Nephi 26:12 "And as I spake concerning the convincing of the Jews, that Jesus in the very Christ, it must needs be that the Gentiles be convinced also that Jesus is the Christ, the Eternal God;"
2 Nephi 30:2 "For behold, I say unto you that as many of the Gentiles as will repent are the covenant people of the Lord; and as many of the Jews as will not repent shall be cast off,"
2 Nephi 31:17 "For the gate by which ye should enter is repentance and baptism by water; and then cometh a remission of your sins by fire and the Holy Ghost."
This is nothing short of eisegesis.
The mystery (μυστήριον, an ordinance/covenant whereby Jews and Gentiles are promised that if they are faithful they will be heirs along with Christ) is not synonymous with the Gospel (the good news about Christ). Knowledge about Christ nor that the Gentiles would receive the covenant was not new with Paul (James quotes an OT prophecy that the Gentiles would come around in Acts 15 [Amos 9:11]). The new part of Paul's revelation appears to be a change in how Gentiles were to receive the covenant. Prior to Christ (and Peter and Paul) they had to be adopted into the covenant which included following Law of Moses principles like circumcisions. Christ fulfilled that Law and introduced or restored a new set of ordinances (baptism, temple endowment). It would be difficult for Paul to be able to know what prophets of the past knew where the scriptures he had access to were silent (missing precious parts).
That the Gentiles would be included in the (new) covenant people of God formed an important part of Old Testament theology. Apart from Amos 9:11, mentioned above, the prophet Isaiah made many explicit prophecies of such, including:
And the foreigners who join themselves to the Lord, to minister to him, to love the name of the Lord, and to be his servants, all who keep the sabbath, and do not profane it, and hold fast my covenant--these I will bring to my holy mountain, and make them joyful in my house of prayer; their burnt offerings and their sacrifices will be accepted on my altar; for my house shall be called a house of prayer for all peoples. (Isa 56:6-7 NRSV; cf. 66:18-22).
Commenting on this prophecy, one scholar wrote the following:
In the Synoptic accounts of the temple cleansing in Matthew 21:13, Mark 11:17, and Luke 19:46, Christ refers to God's house being a house of prayer. That reference to "house of prayer" occurs in Isaiah 56:6-7, where the prophet, Third Isaiah (as we commonly call him), foresaw major changes in the temple liturgy in the future, changes so major that Levitical priests would not be the only ones offering sacrifices. In Isaiah 56:6-7, God announces through the prophet that foreigners will offer burnt offerings and sacrifices in his holy mountain (where the Jerusalem temple is located) and that God will accept them on his altar. This is looking beyond priesthood confined to the tribe of Levi. It is looking forward to something major happening in the future that will involve a massive change in the temple liturgy and the Levitical priesthood. The shock in Isaiah 56:6-7 is its prediction that foreigners will come to minister in the temple, because the word used for minister/serve in 56:6, šārat (שׁרת), typically refers to liturgical service. The prophet sees Gentiles offering sacrifices in Jerusalem. This is omitted from Isaiah 56 in the Dead Sea Scrolls, perhaps because this idea was so repugnant. It is highly suggestive that as Christ cleanses the temple he quotes part of a Scripture passage referring to foreigners undertaking priestly sacrificial duties in the temple. (Thomas J. Lane, The Catholic Priesthood: Biblical Foundations [Steubenville, Ohio: Emmaus Road, 2016], 32)
There is no true contradiction between the Bible and the Book of Mormon on this point. As usual, Sandra Tanner embarked on an exegetical fishing trip, but not only forgot to pack the fishing rods, she did not even know how to fish to begin with.