Rhetorical: Nephi now turns the evidence to the personal, and uses the accepted belief in these traditional prophets as the condemnation of the people. The tells them that “ye know these things.” Nephi has been citing prophets that they continue to accept, even though they are obviously in a form of apostasy. By beginning with this acceptance, Nephi has shown them that they should also accept the Messianic teachings of these prophets.
When Nephi says that “ye have rejected all these things,” he is not referring to the tradition of the prophets, as they clearly still accept that. He is referring to the portions of the prophetic tradition that refer to the coming Atoning Messiah.