Samuel is speaking to Nephites who should know about the Atoning Messiah. They should know about this conditional repentance. They have left that teaching in favor of the Nehorite gospel, but it hasn’t been that long since the true gospel has been taught among them. It is this knowledge from prior to their apostasy that brings them to the condition of “knowing these things.” Samuel fears that because they have known, and now reject these teachings, that they will “come under condemnation.” The Lamanites who had never known of the Atoning Messiah are not under condemnation for not believing, but the Nephites have no excuse, and do fall under condemnation for not following the knowledge they have received.