Act I: The 4th century AD. Valens, the Roman governor of Antioch, issues a decree that in honour of Diocletian's birthday all citizens will offer sacrifice to Venus, the Roman goddess of love, and Flora, a fertility goddess of the spring, on pain of death, and puts Septimius in charge of enforcing this. Didymus, a soldier secretly converted to Christianity, asks that citizens whose consciences prevent them making sacrifices to idols be spared punishment, which Valens dismisses. Septimius suspects Didymus is a Christian and affirms his own loyalty to the law although he pities those who will be condemned to die by the decree and wishes he could be allowed to extend mercy to them. Theodora, a nobly-born Christian and her friend Irene are worshiping with their fellow believers in private rather than joining in the festival for the emperor's birthday when a messenger brings news of Valens' decree. Septimius comes to arrest them - Theodora expects to be put to death but is informed that instead she has been sentenced to serve as a prostitute in the temple of Venus. Theodora would much have preferred to die, but is led away to the temple. Irene informs Didymus who goes in the hope of either rescuing her or dying with her. The first Act closes with a chorus of Christians praying for the mission's success.