Antionah, a chief ruler among them, erroneously thought he had refuted by the scriptures (v. 21; see also Genesis 3:24), the possibility of immortality. Alma’s answer is enlightening. The partaking of the forbidden fruit caused the fall of Adam and all mankind. A physical change in the body was the result. President Joseph Fielding Smith explained: “After the fall, which came by a transgression of the law under which Adam was living, the forbidden fruit had the power to create blood and change his nature and mortality took the place of immortality, and all things, partaking of the change, became mortal.” In this fallen state, mankind is subject to temporal death (Alma 12:24), but in the wisdom of God, this was not to happen immediately. Therefore, Adam was commanded to not eat of the tree of life that would have brought another change. This change would have overcome death and made Adam an immortal being again and there would have been no further change throughout eternity (v. 23). In this immortal state there would have been no children. Thus, “Adam fell that men might be,” but in God’s plan, “men are that they might have joy” (2 Nephi 2:25). Therefore, as Alma explained, “there was a space granted unto man in which he might repent” (Alma 12:24) and receive that joy. The fall had also made mankind a lost people as well as fallen (v. 22). All men would sin “and come short of the glory of God” (Romans 3:23). The agency of man requires choices, or in Alma’s words “a probationary state.” Mortality thus has two purposes: first, to prepare for the time when man would die and “are taken home to that God who gave them life” (Alma 40:11); and secondly, “to prepare for that endless state … which is after the resurrection of the dead” (Alma 12:24). Those who have prepared their body for a celestial resurrection will be quickened by the celestial glory, and those who are not prepared to abide a celestial glory will receive the glory for which they are prepared, either a terrestrial or a telestial glory (see D&C 88:14–24). All of these resurrections will be according to the “plan of redemption which was laid from the foundation of the world” (Alma 12:25).
Returning to the original question asked by Antionah, cherubim and a flaming sword were placed to prevent Adam and Eve from eating of the fruit of the Tree of Life and making them immortal so they could experience the probation state. The absence of a probationary state was not according to the plan of redemption or the word of God (v. 26).