The article below on Parthian Religion is authored by Professor M. Boyce and was originally posted in the Encyclopedia Iranica).
Nothing is known of the religion of the Parni before they entered Parthia, but it seems likely that it was essentially the ancient Iranian polytheism, perhaps already influenced by Zoroastrianism. The immigrants are known to have adopted the Parthians’ language, and with it they presumably took over elements of their culture, including their more evolved, Zoroastrian religion. Since, moreover, it is politically expedient for ruler and ruled to be of one faith, it may reasonably be assumed that, at least from the time they seized power, the Arsacids were professed Zoroastrians.
Evidence concerning their religion remains scanty, considering the length of their rule. It is possible nevertheless to trace some important developments in observance, notably in the fire cult. Temple fires had been established only late in the Achaemenian period, and it is possible that the Parthian sacred fire of Ādur Burzēn-Mihr (q.v.) was the first one to enjoy more than local fame. It is likely that the Arsacids deliberately promoted its legendary sanctity and encouraged pilgrimage to it, as later Shah ʿAbbās encouraged pilgrimage to Mašhad in the same region, for religious, political, and economic motives. Further, the first known regnal fire seems to be that recorded by Isidore of Charax (Parthian Stations 11): “Beyond is Astauene…and the city of Asaak, in which Arsakes was first proclaimed king; and an everlasting fire is guarded there.” The custom of establishing a temple fire at a king’s coronation appears to have been a regal development of the age-old one of a new householder kindling his hearth fire; and it spread under the Arsacids to their vassal-kings (see Nāma-ye Tansar, ed. M. Mīnovī, Tehran, 1932, p. 22, tr. M. Boyce, Rome, 1968, p. 47 with pp. 16-17).
Another Arsacid development of the cult of temple fires was perhaps that of endowing such a fire for the soul (pad ruwān) of an individual. This development is in accord with traditional Zoroastrian care for the soul (which was held to benefit from the merit of the fire’s consecration and that of all pious acts performed for it thereafter); but it could not have taken place before the institution of temple fires had become well established. The earliest evidence relating possibly to such foundations comes from ostraca excavated from the Arsacids’ first capital of Nisa (q.v.), and relates to deliveries of goods from estates which formed part of some royal endowment. The kings concerned are Priapatius (ca. 191-76 B. C.), Mithradates I (ca. 171-38), Artabanus I (ca. 127-24/3), and Gotarzes I (ca. 90-78). In the case of the last-named, the record comes from his lifetime; and it may be that all the foundations concerned were made by the kings for their own souls (a pious custom attested also in the Sasanian period; see I. M. D’yakonov and V. Livshits, Dokumenty iz Nisy, Moscow, 1960, pp. 20-21; A. Perikhanian, VDI, 1972 , pp. 12-13). Arsacid Nisa provides the oldest term known for a priest tending a sacred fire, namely ʾtwršpt “master of a fire” (cf. Av. atarš, nom. sing. of ātar- “fire” and °pati- “lord [of]”). The Western Iranian title magus (spelt mgwšh) for priest also occurs, suggesting the spread of a common terminology among Zoroastrian communities in Achaemenian times. The Nisa ostraca show further that the Arsacids continued the Achaemenian innovation of shrines dedicated to particular divinities.
Another Achaemenian practice adopted by the Arsacids (and continued by the Sasanians) was that of embalming the bodies of kings and laying them in mausoleums. The royal tombs are said by Isidore (Parthian Stations 12) to have been at Nisa. (On the misconception that later Arsacids were entombed at Arbela see J. Hansman in this Encyclopaedia, under Arbela.) The practice conformed, though elaborately, to the Zoroastrian law that the earth must be protected from the impurities of a corpse. That the Arsacids’ subjects widely practiced exposure of the dead is attested by Pompeius Trogus (apud Justin 41.3)
The Nisa ostraca also show that the Arsacids used the Zoroastrian calendar (created under the Achaemenians, probably in the fourth century B.C.), in conjunction with their own era, in daily life. The earliest ostracon so dated belongs to 90/89 B.C. (D’yakonov and Livshits, Dokumenty, p. 69 no. 16; Corp. Inscr. Iran. II/II, Texts I, p. 33 no. 294). Arsacid use of this calendar is further attested by the Parthian legal document from Awromān (q.v.; “year 300, month of Arwadād [ʾrwtt],” see Henning, “Mitteliranisch,” p. 29), and an inscription of Ardabān V (“year 462, month of Spandārmad (spndrmty), day of Mihr,” see W. B. Henning, Asia Major, N. S. 2, 1952, p. 176.
One of the Greek Awromān documents (E. H. Minns, “Parchments of the Parthian Period from Avroman in Kurdistan,” Journal of Hellenic Studies 35, 1915, pp. 28 [Gk. text] and 31 [translation]) establishes moreover that the Arsacids practiced the Zoroastrian custom of close-kin marriage (xwēdōdah, q.v.), a custom also well-attested among their subjects.
In A.D. 62 the Arsacid king Vologases I put his younger brother Tiridates on the throne of Armenia. Tiridates was noted for his strict piety; and under him and his descendants Armenia became predominantly and devoutly Zoroastrian (see Armenia, religion.).
Either this Vologases (Valaxš), or one of the other Arsacid kings of that name, is honored in Zoroastrian tradition for taking measures to preserve “in each province whatever had survived in purity of the Avesta and Zand, as well as every teaching derived from it… whether written or in oral transmission” (Dēnkard IV, p. 412, tr. M. Shaki, Archiv Orientalní 49, 1981, pp. 114-25). A coin of Vologases IV (A.D. 147-91) has on its reverse a fire-holder, which, it has been suggested, may indicate a strengthening of the iconoclastic movement which was to triumph under the next dynasty. (See Iconoclasm, Zoroastrian.)
The Arsacids maintained in general the Achaemenian tolerance regarding the beliefs of non-Iranians; but during their epoch Jewish, Christian, and Buddhist proselytizing gathered strength, and the need for Zoroastrianism to defend itself among its own people, the Iranians, clearly contributed to the change to greater harshness which characterized their successors, the Sasanians.
See also for the Nisa documents Ph. Gignoux, Glossaire des inscriptions pehlevies et parihes, Corp. Inscr. Iran., Supplementary Series I, London, 1972, p. 43 (bibliography), 45-68 (glossary). I. M. Diakonoff and V. A. Livshits, Parthian Economic Documents from Nisa, ed. D. N. MacKenzie, Corp. Inscr. Iran. II/II, London, 1976-79, Plates I-III; London, n.d., Texts I (with complete bibliography).