Samuel is speaking to Nephites who should know about the Atoning Messiah and the conditions required for repentance. They have turned from that doctrine to embrace the Nehorite heresy, but their pre-apostasy knowledge means that Samuel can confidently say they “[know] these things.” Samuel fears that, because they have known and now reject these teachings, they will “come under condemnation.” The Lamanites who had never known the Atoning Messiah (or not known about him for nearly 600 years) are not under condemnation for not believing, but the Nephites have no excuse and are under condemnation for not following the knowledge they have received.