Sherem realizes that he is about to die, and asks that the people be gathered. He had preached publicly, therefore he had to make amends publicly. He had asked for a sign and that sign had been given. He was soon to face the being whom he had denied, and now knew He had power. Therefore, he atones for his own actions by affirming the coming atoning Messiah whom he had previously denied.
Sherem also realized that since Jehovah had stricken him nigh to death, that his soul was also in danger. Thus, he worries about hell and punishment, and is concerned that he would not be forgiven. Modern Latter-day Saint teachings speak of an unpardonable sin, but even with his denial, Sherem does not meet the modern definition of the term. For Sherem, it is simply wondering if he could be pardoned. Since he had not learned about the atonement, and had actively denied it, he would not have understood the expanse of God’s mercy.