Isis: Primal Maternity
The cult of Isis was centered around the Egyptian deities Isis and Osiris. Its Roman origins are placed in the early empire, which would make it very likely that the Egyptian campaigns of the Second Triumvirate (most likely under Marcus Antonius) brought the cult to Rome. It had a popular following up until the fourth century CE, but as Christianity took a firmer hold was completely eliminated by the sixth century.
The central image of the cult concerned the myth of the death and rebirth of Osiris. In this myth, Osiris' brother Seth (god of death and punishment) was envious of Osiris' rulership of Egypt and the Nile and murdered him, cutting him into many pieces. Osiris' wife Isis then gathered the pieces together and took them into herself and gave birth to Osiris, resurrecting him. Another common version states that she could find every part but his phallus, and he was then born without one. While the rituals of the cult remain a mystery even today, we do know that in the initiation ritual the initiate experienced symbolic "death" and "rebirth," purified by the Goddess Isis. Through this the individual would gain access to revelations and devine secrets. The best evidence we have lies in Apuleius's stories entitled the Golden Ass. In these stories he describes his attraction to the Isis cult and eventually reveals that he will be reborn in death as a clean and new person.
The cult mainly concerned itself with concepts of material sacrifice (such as fasting and donations of wealth), and rituals involving symbolic death and the revelations of cult secrets. Unlike other mystery religions, there were both yearly rituals and daily services. Not only this, but there was a very public display of devotion to Isis, with temples, devoted priests, and the worshipers in her honor. Most evidence suggests that the cult of Isis was the largest competitor for Christianity in the Empire, especially concerning the inclusion of women as priests and worshipers equally. That it had an influence on the budding religion of Christianity can be in little doubt, for the iconography of the Virgin Mary often bears similarity to Isis, and Isis' titles such as the "Queen of Heaven" are still used to refer to the Virgin even today. Finally, Osiris's promise of eternal life (through his sacrifice) for his followers has clear parallels to early Christian understandings of Christ.
The Internet Ancient History Sourcebook
Mystery Religions of the Roman Empire: Isis Mysteries
Mythology, Edith Hamilton