Alma was speaking of the First Resurrection in relation to earthly time. Jesus Christ would be resurrected first, followed shortly thereafter by the righteous who had lived and died from the beginning of time for our mortal earth down to the time of Christ’s Resurrection (see Alma 40:16, 20; D&C 133:54–55). This resurrection is what Alma called the First Resurrection.
President Joseph Fielding Smith explained that the First Resurrection includes different time periods and events:
“While there was a general resurrection of the righteous at the time Christ arose from the dead, it is customary for us to speak of the resurrection of the righteous at the Second Coming of Christ as the first resurrection. It is the first to us, for we have little thought or concern over that which is past. The Lord has promised that at the time of his Second Advent the graves will be opened, and the just shall come forth to reign with him on the earth for a thousand years. …
“At the time of the [second] coming of Christ, ‘They who have slept in their graves shall come forth, for their graves shall be opened; and they also shall be caught up to meet him in the midst of the pillar of heaven—They are Christ’s, the first fruits, they who shall descend with him first, and they who are on the earth and in their graves, who are first caught up to meet him; and all this by the voice of the sounding of the trump of the angel of God’ [D&C 88:97–98]. These are the just, ‘whose names are written in heaven, where God and Christ are the judge of all. These are they who are just men made perfect through Jesus the mediator of the new covenant, who wrought out this perfect atonement through the shedding of his own blood’ [D&C 76:68–69].
“Following this great event, and after the Lord and the righteous who are caught up to meet him have descended upon the earth, there will come to pass another resurrection. This may be considered as a part of the first, although it comes later. In this resurrection will come forth those of the terrestrial order, who were not worthy to be caught up to meet him, but who are worthy to come forth to enjoy the millennial reign” (Doctrines of Salvation, comp. Bruce R. McConkie, 3 vols. [1954–56], 2:295–97).
Elder Bruce R. McConkie (1915–85) of the Quorum of the Twelve Apostles gave the following explanation regarding the First Resurrection, which is also referred to as the Resurrection of the Just and as the Resurrection of Life: “Those coming forth in the morning of this resurrection do so with celestial bodies and shall inherit a celestial glory; these are they who are Christ’s the firstfruits. Those coming forth in the afternoon of this resurrection do so with terrestrial bodies and consequently shall inherit that kingdom; they are described as being Christ’s at this coming. All who have been resurrected so far have received celestial bodies; the coming forth of terrestrial beings does not commence until after the Second Coming. (D.&C. 76:50–80; 88:95–99.)” (Doctrinal New Testament Commentary, 3 vols. [1971–73], 1:196).