As one looks upon a field of grain, it is easy to tell that the time has come for harvest. Ammon states that the field was ripe but it must not have looked that way when they started on their journey. Rather, to mortal eyes the Lamanite field would have appeared to be a vast wasteland of weeds, briers, and thorns. Why else would Ammon's friends have laughed him to scorn when they told them of their plans to do missionary work among the Lamanites (v. 23-24).
Thus, we see that the Lord in his omniscience knows when the field is ripe. We don't. We should never say, "there is no hope with this person or with this group of people." Through spiritual eyes, we may be surprised to see how ripe the field really is. Even if the task seems as formidable as preaching to the ancient Lamanites, the Lord has promised us that the people of the last dispensation are ready for the gospel, for he has said, behold the field is white already to harvest; and lo, he that thrusteth in his sickle with his might, the same layeth up in store that he perisheth not, but bringeth salvation to his soul (DC 4:4).