It seems apparent that Alma was told during his crash training program in the spirit world that every one gets a chance to either accept or reject the gospel, either in this life or in the spirit world. He also learned that the wicked who reject the gospel will suffer their portion of God's everlasting wrath until they have paid the uttermost farthing for their sins.
What a testimony Alma and the four repentant sons of Mosiah had to tell! God had saved them by sending an angel. As a result they started on what would turn out to be a lifetime mission of preaching the gospel.