How are claims to God’s guidance to be understood against the background of fears, fundamentalism, and violence inspired by religious belief? But equally, how are acts of humanity, love, and sacrificial service to be understood, when they also claim to be inspired by God? How is healthy religion to be distinguished from unhealthy religion? Questions like these were the subject of lively debate in the first-century world of Corinth, where the views of Greek, Roman, Jewish, and early Christian residents mixed continually, and where Paul established one of the first Christian communities. While their differences were real, there was also common ground and a shared critique of destructive religion. This study looks at how believers and unbelievers confront questions about divine guidance, discernment, delusion, and rational thought. Part I looks at Greco-Roman views, focusing on the archeology of ancient Corinth and the writings of Homer, Virgil, Lucretius, Posidonius, Cicero, Seneca, Plutarch, and others. Part II surveys Jewish attitudes by looking at Philo and Josephus, Qumran, early rabbinic writers, and other intertestamental literature. Part III unpacks Paul’s First Letter to the Corinthians to show that issues of divine guidance and discernment are woven throughout as Paul shapes a distinctly Christian approach. Part IV brings the historical strands together and considers religious experience research to draw some conclusions about discernment and delusion today in the hope that rational and mystical need not be mutually exclusive.