Senate Octavian "Augustus" Princeps Romanization "terrible third century" Severan rulers Germanic invasions Christianity absolute monarchy gospel economic trend Livy History of Rome Jews and Gentiles senatorial class equestrian class Septimius Severus paterfamilias barbarians as mercenaries population declined Natural rights gladiatorial shows Julio-Claudian emperors Jesus Paul of Tarsus praetorian guards China rich and poor Upper-class Roman women