By “these things,” Mormon means the future judgment of the world by the two sets of twelve chosen witnesses. Mormon is beginning with his testimony about his purpose in writing. He explains, “And for this cause I write unto you, that ye may know that ye must all stand before the judgment-seat of Christ.” He knows that all will be judged and drives home to his future readers that they, too, will stand before their judges. Mormon writes so that we will be prepared for that great, awful, and awesome day.