Nephi strengthens his position by invoking Abraham as well. Nephi is using the ties to the venerated prophets of old to bolster his present position. Since Nephi as a prophet of the traditional Nephite religion represents the religion of the Atoning Messiah, this is the issue that he uses as a contrast to the Gadianton secularism. By linking the revered patriarchs to the message of the Atoning Messiah, Nephi ties this belief to revered antiquity and reminds them that they should return to that belief.
This line of reasoning that Nephi is tracing here is designed to show the importance of the prophecy of the coming of the Atoning Messiah. Mormon has not given us any indication to this point that the rejection of the prophecies of the coming of the Messiah was an important aspect of the Gadiantons, but we will see that it will come. Nephi is responding to this unstated aspect of the Gadiantons.