Complex sentence: Rather he advances the theory that "Nazarene" should be understood as an insulting epithet, an unflattering reference to Jesus' humble and obscure origins that was used by anti-Christians at the time.
Split into simpler sentences: Rather he advances the theory that Nazarene should be understood as an insulting epithet. A unflattering reference to Jesus' humble and obscure origins that was used by anti-Christians at the time.