Critics argue that Alma appears to be ignorant of Lehi's 600-year prophecy of Christ's coming (1 Nephi 10:4) since Alma says in Alma 13:25, "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."
According to Matt Roper, Alma is not speaking here of Jesus' birth--of which he already knows--but of Jesus' coming among the Nephites in their own land. In fact, Alma says nothing about Christ's birth in this passage, but speaks of the Lord's "coming in his glory." An interesting phrase. On the small plates, Nephi had foretold that, at some unspecified time "after Christ shall have risen from the dead," he would show himself unto the Nephites, "and the words which he shall speak unto you shall be the law which ye shall do" (2 Nephi 26:1). Contrary to what critics might think, Alma 13 is consistent with Nephi's earlier prophecy on the small plates. Alma states that angels had already begun the work of preparing the Nephites to receive Christ's teachings at the anticipated time of his visit among them.
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 his word at the time of his coming in his glory [that is, among the Nephites]. And now we only wait to hear the joyful news declared unto us by the mouth of angels, of his coming [that is, among the Nephites in their own land]; 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. (Alma 13:24-25)
The prophecies on the small plates would have told of the date of Christ's birth, but would not have told the date of his death or exactly how long after the resurrection Christ would appear to the Nephites. It is clearly that great day which Alma longs to see. Obviously Alma was familiar with the prophecies which spoke of his coming among the Nephites. He declared, "that the words of our fathers may be fulfilled, according to that which they have spoken concerning him, which was according to the spirit of prophecy which was in them" (Alma 13:24; cf. Alma 5:50-52).
He also taught his son Corianton, somewhere around 73 B.C., that they were "called to declare these glad tidings unto this people, to prepare their minds . . . that they might prepare the minds of their children to hear the word at the time of his coming" among them (Alma 39:16). Alma wanted to prepare the people in his land, so that they would prepare their children for Christ's coming among them. So Alma appears to have known that Christ would not come in his lifetime, but in the lifetime of at least some of the rising generation, information which, it is reasonable to assume, he learned from the records in his possession. Mormon had stated a few years earlier, "And many of the people did inquire concerning the place where the Son of God should come; and they were taught [by Alma who was the record keeper of the small plates?] that he would appear unto them after his resurrection; and this the people did hear with joy and gladness" (Alma 16:20).
It is interesting to note that Alma was probably fairly young between 100 and 92 B.C. If he could have lived until, say, 32 B.C., it would be well within the range of reasonable interpretation for a general six-century estimate for Christ's birth (as opposed to an exact 600-year prophecy). But old age was not something one could take for granted. And besides, Christ would have to live a number of years before he started his ministry and ultimately came to the Nephites. This might explain Alma's somewhat wistful hope that Christ might come--though probably not in his own time.
But one might ask, Why would Benjamin and Alma not speak more specifically of the date of Christ's birth and Lehi's 600-year prophecy in their public discourses in the land of Zarahemla? Perhaps this information was considered a sacred mystery reserved for the faithful. Nephite prophets often concealed certain scriptural information from the public at various times in their history, for diverse reasons (see Alma 37:27-29; 45:9; 3 Nephi 28:25; Ether 4:1). It might be suggested that Samuel's prophecy was considered significant and unique because it was the first public disclosure of the date of Christ's birth among the people of Zarahemla and not because the information was new. The largely negative reaction of the people (Helaman 16:6-23; 3 Nephi 1:4-10) is reason enough for the prophets to have concealed the information so long. [Matt Roper, "A More Perfect Priority?," in Review of Books on the Book of Mormon, Vol. 6, Num. 1, pp. 364-366]