Alma explains the solution: the atonement. It resolves the problem of temporal death—which is universal for all human beings—by assuring their resurrection.
Alma’s discussion up to this point has simply covered up to his original statement: that a man might live forever after the resurrection. Antionah had emphasized the problem of sin, a natural question for a Nehorite, as one tenet was to deny that sin required personal repentance and atonement.