Just as the Old Testament Prophets knew of Jesus Christ, so did the Book of Mormon Prophets. As Jacob testifies:
For, for this intent have we written these things, that they may know that we knew of Christ, and we had a hope of his glory many hundred years before his coming; and not only we ourselves had a hope of his glory, but also all the holy prophets which were before us.
Behold, they believed in Christ and worshiped the Father in his name, and also we worship the Father in his name. And for this intent we keep the law of Moses, it pointing our souls to him; and for this cause it is sanctified unto us for righteousness, even as it was accounted unto Abraham in the wilderness to be obedient unto the commands of God in offering up his son Isaac, which is a similitude of God and his Only Begotten Son.
We can take comfort in the fact that not only does Jesus Christ work among Old and New Testament Prophets, but also among His “other sheep” in ancient America.
Source: Jacob 4:3-4; The Book of Mormon: Another Testament of Jesus Christ