According to Bruce Warren, although we may not know the identities of all the "other sheep" to which Christ makes reference to in 3 Nephi 15:21, we do know that there was a great intelligent population of Nephites in the Americas. Moreover, there is ample evidence that at least certain Palestinian individuals had an understanding of these Mesoamerican peoples. Religious scholar and philosopher Hugh Nibley has written about the very "interesting hint" dropped by Origen:
Clement, the disciple of the Apostles, recalls those whom the Greeks designate as antichthonians [dwellers on the other side of the earth], and other parts of the earth's sphere [or circuit] which cannot be reached by anyone from our regions, and from which none of the inhabitants dwelling there is able to get to us; he calls these areas "worlds" when he says: "The Ocean is not to be crossed by men, but those worlds which lie on the other side of it are governed by the same ordinances [lit. dispositions] of a guiding and directing God as these."
Nibley continues:
Here is a clear statement that the earliest Christians taught that there were people living on the other side of the world who enjoyed the guidance of God in complete isolation from the rest of the world. Origen knows of mysterious knowledge that was had among the leaders of the Primitive Church but was neither divulged by them to the general public nor passed on to the general membership, and this includes the assurance that there were people living on the other side of the world who enjoyed the same divine guidance as themselves in a state of complete isolation (Nibley, An Approach to the Book of Mormon, 1988, 326).
[Bruce W. Warren, Blaine M. Yorgason, Harold Brown, New Evidences of Christ in Mesoamerica, Unpublished Manuscript]