Alma lays out two conditions for obtaining eternal life: be baptized (“whosoever doeth this,” referencing baptism), and keep the commandments. This is the same commandment we see in Matthew 10:22: “he that endureth to the end shall be saved.”
These are the conditions. Alma asks the rhetorical question of whether they believe them. It is rhetorical because the Spirit had already testified to him that they did. When Alma began the sermon, he noted that he had joy in those of Zarahemla who had repented, but in Gideon the joy is “because your faith is strong.” They are not in the state of apostasy like those in Zarahemla.