Terrance Szink notes that in a way reminiscent of evil Haman's actions in the book of Esther, the wicked among the Book of Mormon people set a day for the death of the believers if the sign accompanying the birth of Jesus ("that day and that night and that day which should be as one day"--3 Nephi 1:8) were not fulfilled. . . . The night that the sign was given there was an astonishment throughout all the land. So overwhelming was the power of this sign that for the time being "the more part of the people did believe, and were converted unto the Lord" (3 Nephi 1:22). Though their conversion may have been genuine at that time, it did not last long. The passing of only three years found most of the people "hard in their hearts, and blind in their minds, and [they] began to disbelieve all which they had heard and seen." (3 Nephi 2:1.)
On the surface it is difficult to see how the people could fall so quickly after being converted (see Sperry, The Book of Mormon Testifies, p. 286). As mentioned above, the conversion was the result of an outward sign and required no work on the part of the people. They had seen the sign but, for the most part, they did not sustain their faith.
It is ironic, however, that although they did not believe the sign, "the Nephites began to reckon their time from this period when the sign was given, or from the coming of Christ." (3 Nephi 2:8) They had previously marked time from when Lehi and his family had left Jerusalem and later from the start of the reign of the judges. This is perhaps analogous to the situation in the western world where time is also calculated from the birth of Christ and yet a great number of people do not believe in Jesus as the Son of God. [Terrence L. Szink, "A Just and a True Record," in Studies in Scripture: Book of Mormon, Part 2, pp. 126-128]