Franz Cumont 
Oriental Religions in Roman Paganism [PDF ebook] 

This study by the great Belgian historian Franz Cumont describes one aspect of the cultural meeting of east and west in the early Roman empire. It describes the great pagan religions of the orient, and tells how their religious thought and ceremonies permeated, altered, and revivified Roman paganism.
It provides a coverage of all the more important eastern religions of the time, from their first appearance in Rome, 204 B.C., when the great Mother of the Gods was first imported from Syria:
The ecstatic cults of Phrygia and Syria; the worship of Cybele, the Magna Mater, Attis, Adonis; their orgies and mutilatory rites. The mysteries of Egypt; the worship of Serapis, Isis, Osiris, their closely hidden secret rites, redemption ceremonies.
The dualism of Persia; the elevation of cosmic evil, to a full and equal partnership with the deity; the mysteries of Mithra. The worship of Hermes Trismegistos, and the documents ascribed to him; Sabazios, Ishtar, Astarte.
The magic, thaumaturgy, judicial astrology of the ancient near east.
The emotional and intellectual impact of the great civilized traditions of Egypt and Babylonia upon still barbarian Europe.
Cumont’s ‘Oriental Religions in Roman Paganism’ is the best general picture, on an intermediate level, of this important moment in cultural history. It is also of great value in analyzing an era which shared certain cultural problems with our own time.