In Alma 13:21-26, Alma2 states the following:
Now is the time to repent, for the day of salvation draweth nigh; Yea, and the voice of the Lord, by the mouth of angels doth declare it unto all nations; yea, doth declare it, that they may have glad tidings of great joy; yea, and he doth sound these glad tidings among all his people, yea, even to them that are scattered abroad upon the face of the earth; wherefore they have come unto us. and they are made known unto us in plain terms, that we may understand, that we cannot err; and this because of our being wanderers in a strange land; therefore, we are thus highly favored, for we have these glad tidings declared unto us in all parts of our vineyard. For behold, angels are declaring it unto many at this time in our land; and this is for the purpose of preparing the hearts of the children of men to receive this word at the time of his coming in his glory. And now we only wait to hear the joyful news declared unto us by the mouth of angels, of his coming; for the time cometh, we know not how soon. Would to God that it might be in my day; but let it be sooner or later, in it I will rejoice.
Book of Mormon critics find it unreasonable that the prophet Alma, who lived hundreds of years after Lehi's 600-year prophecy of Christ's birth as recorded by Nephi in 1 Nephi 10:4, seems to know nothing about the 600-year prophecy. However, according to Matthew Roper, one may reach several different conclusions depending on how one interprets the references to Christ's "coming" in Alma 13:24-26
1. Christ's Birth: John Tvedtnes has argued that Alma and perhaps other Book of Mormon prophets mentioned in the books of Mosiah through Moroni may have been unfamiliar with the prophecies on the small plates of Nephi. Alma had a voluminous library of scriptural records, far beyond anything we have today--not merely a few books. . . . This would be a fairly imposing corpus to read, much less to master and remember. Previous to Alma's conversion he had been a very wicked and idolatrous man (see Mosiah 27:8-10). It is doubtful that he spent much time at that earlier period searching the scriptures. He received the library of records and other sacred things from King Mosiah2 only shortly before becoming the first Nephite chief judge (Mosiah 28:20). . . . Is it really unreasonable that Alma did not spend much time on this little record of Nephi until several years after receiving them? One may, therefore, reasonably interpret Alma's statement as a reference to Alma's ignorance of the 600-year prophecy at the time of his mission in Ammonihah during the tenth year of the reign of the judges. It seems clear that Alma was familiar with at least some of the prophecies on the small plates by at least the fourteenth year of the reign of the judges (see Alma 16:15,19-20).
Note* Alma 16:15 says "the curse of God had fallen upon them because of the traditions of their fathers." This curse is referred to in 1 Nephi 2:23; 2 Nephi 5:20-25. Alma also quotes Lehi's exact words from 1 Nephi 1:8 on the small plates in Alma 36:22. [Alan C. Miner, Personal Notes]
2. Christ's Coming to Atone for the Sins of Mankind and Redeem Them from Death: Several statements by Alma regarding the coming of Christ clearly refer to his coming to redeem man from sin and death (see Alma 36:17; Alma 5:48). Thus, there is scriptural precedent for the interpretation that the coming of Christ discussed in Alma 13:25 may refer to Christ's coming to atone for sin and break the bands of death. Some might object to this interpretation on the basis of Alma 13:25-26 in which Alma states that his coming would be declared by "the mouth of angels" . . . however, when Jesus was resurrected many of the departed Saints also arose and appeared unto many of the Nephites and did minister to them. Resurrected beings are certainly angels. Given the time of their appearance, it is reasonable to assume that they announced the fact that Christ's atonement was fulfilled.
3. Christ's Appearance to the Nephites: Tom Nibley suggested that Alma in Alma 13 had reference to Christ's appearance to the Nephites, since he speaks of "the time of his coming in his glory." Even if the term "glory" were not unique to a resurrected appearance, it would still be an appropriate one to describe the Savior's glorious New World appearance.
4. Christ's General Coming: By referring to the coming of Christ, Alma may simply have reference to Christ's coming in a general sense. Under this interpretation, Alma means Christ's coming, beginning of course with his birth, but also including his perfect life and teachings, his atoning sacrifice and death, and his resurrection, culminating for the Nephites in his resurrected appearance to them when he delivered his teachings.
[Matthew Roper, "A Black Hole That's Not So Black," in Review of Books on the Book of Mormon, Vol. 6/2 1994, pp. 180-185]