Isis was one of many non-Greek deities whose cults[Note 1] diffused beyond their home lands and became part of Greek and Roman religion during the Hellenistic period (323–30 BCE), when Greek people and culture spread to lands across the Mediterranean and most of those same lands were conquered by the Roman Republic.[29][30] Under the influence of Greco-Roman tradition, some of these cults, including that of Isis, developed their own mystery rites.[31] Much of Isis's cult involved activities that were far more public than the mystery rites, such as the adoration of cult statues within her temples, or outdoor festivals such as the Navigium Isidis,[32][33] yet scholars often regard the mysteries as one of the most characteristic features of her cult.[34]

The Isis cult developed its mysteries in response to the widespread belief that the Greek mystery cults had originated with Isis and Osiris in Egypt.[9] As the classicist Miguel John Verlsuys puts it, "For the Greeks, the image of Egypt as old and religious was so strong that they could not but imagine Isis as a mystery goddess."[35] Isis's devotees may have adapted aspects of Egyptian ritual to fit the model of the Eleusinian mysteries, perhaps incorporating Dionysian elements as well. The end product would have seemed to the Greeks like an authentic Egyptian precursor to Greek mysteries.[9][36] Many Greco-Roman sources claim that Isis herself devised these rites.[37]