See also: Foreign priesthoods in Nova Roma
THIS NEEDS MAJOR REVISION
As Roman civilization expanded from Italy, the Religio Romana began to be practiced alongside other religions within the various Provinces. This meant that major world religions had to coexist within the same overall society on a major scale for the first time in history. In various lands the Religio Romana was practiced alongside various Greek, Celtic, Spanish, Phoenician, German, Egyptian and Persian religions and cults. The official religion of the Roman state remained virtually unchanged by this (with the exception of cults added to Rome by instruction of the Sibylline Books). However, the overall coexistence of religions in the Roman world created a cosmopolitan society for the Roman Citizenry that is only now starting to be equaled in the modern world.
There were a number of priesthoods of well-established "foreign" cults (such as the cult of Isis and Serapis) that came to be practiced in nearly all of the Provinciae, and within Rome itself. Some of these cults were initiatory, and were called "Mysteries" (such as the Mysteries of Mithras). Other religions were indigenous to one area. These gave birth to Provincial mixes which exemplified both the original faith of local peoples, and also their Romanitas as Citizens of Rome.
As with the Roman deities there are a vast number of Provincial deities that were part of the Religio Romana on a local level only. These priesthoods would include the worship of deities from Provinces including Britannia, Gallia, Germania, Spain, Numidia, etc. The Foreign Sacerdotes are the lesser "everyday" priesthoods that tended Provincial temples and shrines, maintain the worship of a deity, and who assist the populace with their worship of that deity.
The "foreign" cults include religions that came to Rome from other countries and were never completely adopted as part of the official Religio Romana, as well as the cults of deities that were specific to the various Provinciae. Generally such priesthoods are a Sacerdos position, unless there is another specific historical title.
Roman -Persian Cults
Mysteries of Magna Mater
Galii (eunuch-priests-- cannot be Citizens of Rome)
Sacerdotes Magnae Matris (ordinary priests and priestesses)
Mysteries of Isis and Serapis