Perhaps the most influential form of divination to arrive in the Roman world from the East was astrology. Systems for divining the will of the gods and the course of the future emerged in both Egypt and Mesopotamia c. 2000 B.C. and were thus both very ancient when Alexander the Great conquered both in the late fourth century. From there, astrology, practiced by professional experts, moved into the Greek and then Roman world, though Roman elites were often deeply ambivalent about this foreign method of divination; both Cato and Cicero express doubts (of course, the Roman practice of haruspicy was also foreign in that it was Etruscan, but this adoption had been sanctified by long use in Roman tradition and was thus mostly beyond reproach). Nevertheless, it is clear that this form of divination become common, with the writer, geographer and astronomer Ptolemy (c. 100-170 A.D.) even producing a long explication of the practice of astrology in his Tetrabiblos.
This portability is not restricted merely to divination. Herodotus’ suspicion that quite a bit of Greek religion might have come from somewhere else has merit, though Anatolia, not Egypt, appears to be the main source (see: M.L. West, The East Face of Helicon (1999); and for the person already writing this comment, yes I am aware of Bernal’s Black Athena and no I am not convinced, nor are many specialists in the field). The Romans were open about importing gods from Greece and make a clear distinction between gods worshiped in traditional Roman manner and those imported from Greece (a quite small number) and thus whose rituals followed ritus graecus – rituals in Greek fashion.
In other cases, the foreign practice was modified to fit the culture it arrived in. The Romans adopted the cult of Cybele, an Anatolian goddess, during the dark days of the Second Punic War (the Senate made that decision based on a consultation with the Sibylline books, a written source of oracular prophecy we can talk about another day). Cybele was called Magna Mater (“Great Mother”) in Rome, and it seems made some modifications to her rituals, in particular possibly limiting the role of the Galli (eunuch priests) whose rituals and style seemed decidedly “unRoman” (though I should note that the scholarship here is contested and the issue and evidence complex).
The normal technical term for this kind of religious borrowing is syncretism, and it is a sort of interweaving of religious traditions that polytheisms both ancient and modern are exceptionally capable of. It is simply not hard to add one more god or one more ritual into a religious system that already assumes the existence of innumerable gods.
Bret Devereaux, “Collections: Practical Polytheism, Part III: Polling the Gods”, A Collection of Unmitigated Pedantry, 2019-11-08.
