Isis was identified with Hellenic deities such as Demeter or Aphrodite. Greek iconography was introduced to the cult which made it visually appealing to the Hellenes. In those days when the provincial city-states of the Hellenic world fell to Alexander's universal empire, the traditional gods of the city-state no longer sufficed. Gods like Isis and Serapis were not connected with any specific town and were truly universal in scope. More importantly, the exotic Egyptian mysticism could offer the Greeks of the Hellenistic age something their own gods could not - a way to cheat fate and death.
Isis and Osiris were honored by Greeks and by Egyptian emigrants as a kind of holy trinity, but always it was Isis who was the dominant member of the trio. Isis became the protector of family (especially women), the protector of newborns, the goddess of fertility and good fortune, and the goddess whose magic could cheat Fate and Death. She was also thought to be a protector of sailors, and sailors sailing from the great port of Alexandria took her cult all over the Mediterranean. Backed by the Ptolemaic regime, the new cult spread throughout the Hellenistic Kingdoms.
The Nile Flows into the Tiber
The Roman Senate was not amused with Ptolemy's attempt to craft a universal religion.
When the cult of Isis swept into Rome via Hellenistic sailors and Egyptian emigrants, it became outstandingly popular with women and the lower classes, including slaves. Fearing a religious unification of the lower strata of Roman society, and fearing the loss of piety in the traditional Roman gods of the state,
the Senate repeatedly placed restrictions on the new cult.
Private chapels dedicated to Isis were ordered destroyed. When a Roman Consul found that the demolition team assigned to him were all members or sympathizers of the cult and refused to destroy their chapel, he had to remove his toga of state and do the deed himself.
Augustus found the cult "pornographic," though the cult was known to proscribe periods of sexual abstinence to its adherents. The real reason for Augustus' wrath was that the cult was linked to Egypt and thus the power base of his rival, Antony.
Cleopatra had even gone so far to declare herself Isis reincarnated. Nonetheless, Augustus' scorn did little to stem popular opinion. Officials and servants of the imperial household were members of the cult. It seems even his own infamous daughter was a member; whether her belief was genuine or merely another aspect of her defiance against her father cannot be determined.
Tiberius, upon hearing of a sexual scandal involving the cult, had the offenders crucified and images of Isis cast into the Tiber. But much like Christianity, periodic and sporadic persecutions did nothing to stem the tide. What was death when one's deity promised salvation and resurrection?
As part of undoing the policies of Tiberius, Caligula legitimized the religion. Temples to Isis were permitted construction. Aspects of the Isiac festivals became public and part of the civil calendar (though there were still mysteries celebrated in private). It is also known that Caligula had an Egyptian chamberlain who exerted influence on the emperor and helped him progress in the mysteries of the goddess. Perhaps this even helped play a role in Caligula's infamous promotion of himself as an autocratic, Hellenistic-like ruler. Whatever the truth, Isis was now part of Roman paganism for good.
The emperor Vespasian became acquainted with the cult while serving in the Eastern legions, and seems to have adopted Isis and Serapis as his personal savior deities. Domitian owed his life to fleeing opponents in the garb of Isiac cultists, and continued the family's association with the cult.
Hadrian and Marcus Aurelius were friendly to the cult, but most likely not initiates. Commodus, on the other hand, shaved his head bald like the priests of Serapis. He used to beat those around him with a mask of Anubis that was common in the processions of the cult.
Septimus Severus was fascinated with the cult, and his son Caracalla dedicated a giant temple to Serapis that rivaled the one built to Jupiter, Rome's original patron god. The meaning was clear -
the gods of the East that had once been maligned by the ruling classes of the Republic were now on equal footing with the traditional gods of the State. Among the common people, they were more important.
Stoic and Neoplatonic intellectuals tried to reinterpret the cult in terms of their own highbrow philosophies, with the deities of the cult serving as metaphors for great cosmic principles. While this may have held some influence in the literate classes, its doubtful it had any impact on the vast majority of followers. To the average person Isis was not a metaphor or concept; she was as real to her followers as the Virgin Mary, Mother of God, is to billions of Christians around the world today. More to the point, she performed much the same function.
The Un-Roman Roman cult
The Cult of Isis was, thanks to Ptolemy, Hellenized to a degree that the Roman mind could understand it, and yet still foreign enough to be exotic and alien.
Unlike most religious structures in the Roman world, the Iseum did not open to the streets or forum where public spectators could view the proceedings inside. The Iseum was walled off from the surrounding world, suggesting a space of inner sanctity. Even within its walls, there was a "sanctuary" much like modern monasteries where only clergy and the initiated could enter. In there rituals involving fire, water and incense were conducted in front of a sacred statuary of the deities concerned. This secret religious life that was set apart from the community and the State is what helped arouse the suspicions of the conservatives back in the days of the Republic.
Not much is known about the details of the inner workings of the mysteries, as they were by definition secret. Prospective initiates were called to the goddess by dreams and visions. Intense preparations of purification and meditation (and abstinence) were followed by exotic rites designed to recreate the myth of Isis and the resurrection of Osiris. By enduring these rituals, the adherent was reconciled to the magic of Isis and effectively granted a favorable afterlife. He or she was in a sense spiritually reborn in a manner common to Greco-Oriental savior religions.
But there were more public festivals too that didn't require initiation. The first was conducted on March 5th. In honor of Isis sailing the seas to find pieces of her lost husband, a colorful procession of costumed people, including especially sailors, marched to port and ritually blessed a boat. The second festival was held October 28th to November 3rd. This was an ancient passion play Again, costumed enactors took to the streets, this time to reenact the death and resurrection of Serapis. Roman conservatives complained the festival was too loud and colorful.
People also had private shrines to Isis and Serapis in their homes.
The subject of the ethics of the cult is a complicated one. We know that Egyptian culture as a whole was free with sexuality compared to Roman culture. Isis was in fact rather popular with courtesans and other such professions, and there are speculations that Isiac cults may have promoted a kind of "positive sexuality" among a more conservative Roman population. Augustus and Tiberius took this as proof of a "pornographic" cult. Yet the Isiac cult also demanded regular periods of sexual abstinence from its adherents for purposes of ritual purification, and even apparently courtesans readily submitted to these observances. Curiously enough, the early Christians who were quick to complain about the degeneracy of pagan cults could not offer as much criticism about Isis as they could about some other cults in the Empire.
Universal Religion
Unlike Mithraism which was confined to a small percentage of "middle class" Roman males, the Isis cult was truly universal. Unlike Mithraism it could be practiced by both men and women, and it was women who perhaps took it up most enthusiastically. Unlike Mithraism it appealed to all classes; the lower classes and slaves were the mainstay of the cult, but as we have seen even those at the very top of the social strata were also adherents. Unlike Mithraism which was mostly confined to the Latin West,
Isis was honored in both halves of the empire. Isis was long honored in the Greek East, and
penetrated into the Latin West in even barely Romanized areas such as Britain or northwest Gaul. Isis was however
a cult of city dwellers; we see little evidence of Isiac cults in rural areas outside of her native Egypt.
There was little danger of the small cult of Mithras, influential though it was, stemming the tide of Christianity and taking over the world. However,
the cult of Isis had the numbers and the appeal to mount a serious threat to Christianity.
Some scholars assert that the Holy Trinity of Isis, Serapis and Horus were not really defeated - they were merely absorbed into the new Holy Trinity of Christianity. The reverence for Mary among high Christian churches is similar to faith in Isis. We should consider at the very least that many chapels to the Virgin were built purposely on the remains of temples to Isis, and that furthermore the iconography of the Madonna and Christ is quite similar to Isis and Horus.
http://www.unrv.com/culture/isis.php