Religion in the Roman civilization was strongly influenced by the cultures of its primary ‘interest zones’, which in turn seem to have been most strongly influenced by the ‘biogeographical’ zones from which they derived. The progress can be illustrated by the following animated map which dates its territories, influence and influences. You will need to click the map to start the animation! (source: Wikimedia Commons):
The civilization’s origins appear to have arisen from the Etruscan civilisation’s tendency to follow the Greek tendency and to unite the tribal structure of Iron Age Italy under a more centralised civic theocracy – tribal temporal power being united and mobilised through a shared centralised theocracy, based on notions of immanent polytheism (gods presiding over all natural phenomena) then prevalent in the Northern Mediterranean spiritual landscape, particularly in Magna Graecia (which included southern Italy). Success in trade (aided by climate and geography) supported success in the conduct of warfare leading to stability and expansionism, increasingly centred in Latium (the western coastal midlands of Italy) and from around c.500 BCE at its heart in Rome on the river Tiber. This gave rise to the cultural identity we know as ‘Roman’, under a leadership that mutated from Kingship (Res) to Republican rule (Res Publica) with the public aspects of religion overseen and directed by the Patrician class.
The story of this transition was also a story of the transition from conservative agrarian/pastoralist Bronze Age and Iron Age Italic cultures, ruled by the rhythms of nature and sense of place in the landscape, to one focussed on temporal power and influence and creating a new identity giving it a place in the wider scheme of the world. To this end, Roman civilisation began to affect the philosophies, Gods and cultural ideas of its subjects and neighbours. As its reach expanded and these neighbours became its subjects, the process of multi-cultural conflation would become in itself an identifier of Roman culture, and under Roman economic and religious control. Intellectual cultural threats to a spiritual and cultural morass which borrowed rhetoric and ideas from the conquered would be difficult to deflect.
As it distanced itself from the independent tribal style of culture at its origin, and increasingly followed on the footsteps of the Hellenes in its acquisition of oriental territory and ideas, Rome’s religion began to differentiate from that of its Western European origins. It’s ‘frontier zone’ of this difference had, before the 1st century BCE been limited to Gallia Cisaplina (modern North Italy) and Gallia Narbonensis (modern Mediterranean France) which represented the former interface of the Etruscans (and Greeks) with the Gallic tribes further north, and had been the corridor linking its territories in the formerly Celtic provinces of Hispania, the conquest of which were completed in 19BC, and the Romanised peoples of which were to achieve greatness within the Imperium itself (the Senecas, Lucan, and Emperors Trajan, Hadrian and Marcus Aurelius among others). There is no evidence from ancient Roman or Greek writers supporting a doctrine of metempsychosis among the Celtic Iberians, although further west (in Anatolia, now modern Turkey) the Galatians were rumoured to follow the doctrine of druidism, imported from their supposed original homelands in Gallia Narbonensis.
As the 1stC BCE progressed, Rome’s influence would extend from the East to the West of the Mediterranean ‘biogeographical zones’, and its culture had been busy incorporating and transferring religions, ideas and peoples within its boundaries to suit its new multi-ethnic domain. Its attempts to expand into and change the Atlantic part of northern Europe would mark the start of some fundamental changes that would secure the fate of religion in the West for another 2000 years.