The prophets are the first to arise in the resurrection. Even if this is not technically accurate, it is symbolically apt. The prophets are quintessentially righteous. Abinadi emphasizes that there two classes of people will also be saved with the righteous, even though they did not believe.
The first are those who “have died before Christ came” and who were therefore ignorant of his coming mission. This is important, not only because it provides the theological assurance that Yahweh is just and will not condemn his children for what they did not know, but also because Abinadi and the priests are among those who are living before “Christ came.” Abinadi is speaking of people now alive (including his audience) who might be saved. First are the prophets (and by implication those who believe as they do) and second are those who do not believe because they have not heard. All these are saved. Next Abinadi adds “little children.” Once again there is theological comfort for us in knowing this, but Abinadi’s purpose in mentioning this is not to comfort future generations, but rather to condemn a current one.