The Isia was a Roman festival celebrating Isis, the Egyptian goddess whose cult spread throughout the Mediterranean. The multi-day celebration included the Inventio Osiridis (finding of Osiris), mystery rites, lamentations, nocturnal ceremonies, and joyful celebrations of divine resurrection. Devotees wore linen, carried water from the Nile, and participated in emotional ritual dramas. In poetry, Isia represents religious syncretism, mystery cult devotion, and foreign influence on Roman spirituality, appearing in works exploring Egyptian religion's allure, death and resurrection themes, feminine divine power, and Rome's cosmopolitan religious landscape incorporating Eastern mysteries.

Isia brought Egyptian goddess Isis's worship to Rome, featuring mystery rites, processions dramatizing Osiris's search, and emotional ritual performances. The festival's adoption demonstrates Roman religious syncretism and the appeal of Eastern mystery religions. Understanding Isia helps contextualize Roman imperial period religious diversity and the spread of salvation-oriented mystery cults. The festival's elaborate mythology and emotional rituals contrasted with traditional Roman religious restraint, offering worshippers more personal divine relationships and promises of afterlife benefits.