One might wonder why we find the term "Christians" being used in Alma 46:15 in a chronological setting more than a hundred years before Christ began his ministry in the Old World. After all, don't we find in Acts 11:26 that "the disciples were called Christians first in Antioch"?
John Tvedtnes notes that the term "Christ" appears 214 times in the pre-Christian Book of Mormon passages before 3 Nephi. Moreover, the use of the term "Christ" might be justified by the fact that it was the preeminent term for "anointed one" used in Joseph Smith's culture. In other words, in the translation process Joseph Smith used the most appropriate term ("Christians"--Alma 46:15) to express in the best manner possible the idea for a term inscribed upon the plates which represented "those who were true believers in Christ," or true believers in the "Messiah," or true believers in "the Anointed One." [John Tvedtnes, Book Review in Review of Books on the Book of Mormon, Vol. 4 1992, pp. 230-231]
The Book of Mormon student might wonder if this explanation fully correlates with Joseph Smith's usual practice of "spelling out" the names he came across in translating the plates. However, one must realize that it is not specified whether those "spelled-out" names represented the actual Nephite sound or whether they represented the translated true meaning of the original name. What we do know from the Book of Mormon itself is that the name-title "Christ" was conveyed to the Nephites through revelation. In 2 Nephi 10:3 we find:
Wherefore, as I [Jacob] said unto you, it must needs be expedient that Christ--for in the last night the angel spake unto me that this should be his name--should come among the Jews, among those who are the more wicked part of the world; and they shall crucify him--for thus it behooveth our God, and there is none other nation on earth that would crucify their God.
[Alan C. Miner, Personal Notes]
“Christians - Because of Their Belief in Christ Who Should Come”
In Alma 46:15 we find that, "all those who were true believers in Christ took upon them, gladly, the name of Christ, or Christians as they were called, because of their belief in Christ who should come." Richardson, Richardson and Bentley write that the Book of Mormon teaches that the Gospel of Jesus Christ is universal and eternal, and was taught throughout the entire history of man's sojourn upon this earth (except during periods of apostasy) before and after Christ (See Alma 46:15; 48:10).
It is interesting that in the Old World, the so called "first" Christians also taught this idea. They claimed: "that which is called the Christian religion existed among the ancients, and never did not exist from the beginning of the human race." Origen referred to "the Christians that were before Christ." Ambrose said, "We admire the mysteries of the Jews. . . . But I can promise you that the Christian sacraments are both holier and older." (See Seaich, Ancient Texts and Mormonism, pp. 13, 120)
The Gospel of Phillip shows that the word Christ existed before the Christian era by explaining that Christ is the Greek translation of the more general term Messiah. "The name Jesus [Yeshua] does not exist in any other tongue [than Hebrew], but is always called Jesus. But Christ is Messiah in Syriac, while in Greek it is the Christ." (Nibley, Since Cumorah, p. 167)
This idea is also brought out in John 4:25, which quotes a Samaritan woman as saying, "I know that Messias cometh, which is called Christ: when he is come, he will tell us all things." Had there not existed the word Christ as a specific use of the term Messiah then her statement would have been meaningless. [Allen H. Richardson, David E. Richardson and Anthony E. Bentley, 1000 Evidences for the Church of Jesus Christ of Latter-day Saints: Part Two-A Voice from the Dust: 500 Evidences in Support of the Book of Mormon, pp. 30-31]