This particular argument is another contrast. It contraposes the future with the present. The reason is that it argues that there will come a time when the knowledge of the Messiah’s earthly mission will be so well known that all humankind will be responsible for their understanding and application of that atonement to their personal lives.
That future well-known atonement contrasts with the current situation of the people of Zarahemla. It would appear that they are the only ones who have had the teaching, or who have, as a people, accepted it. At this point, some four hundred years after Lehi’s family had left Jerusalem, even though Laman and Lemuel may have originally been taught of the coming Messiah, that understanding had been lost in their descendants, and certainly among all those who are now called Lamanites—peoples who may not have any genetic links with the original family.
Benjamin’s people have been taught, and therefore they are as those of the distant future when all would know of the atoning Messiah. Just as those future people will not be held blameless, unless they accept the atonement and repent, so too the current people of Zarahemla cannot be found blameless, unless they also repent and accept the atonement which is to come.