The cultus deorum Romanorum ('cults of the Roman gods') is a collective term for the various cults and rituals which constituted the religious life of Rome until the edict of Theodosius. The cultus deorum began as the practices of the farmers of the village of Rome.
"I am quite certain that Romulus by instituting auspices, and Numa ritual, laid the foundations of our state, which would never have been able to be so great had not the immortal gods been placated to the utmost extent." -- Cicero "On the Nature of the Gods"
Influenced by their Etruscan, Greek and Phonecian neighbours, the Romans developed a complex state religion that emphasised the patron -client relationship between the gods and Romans: the pax deorum (religio) and maintaining it by scrupulous observance of rituals. 
Roman religion cannot be approached by inserting Roman deity names into Greek religion or any other system, for it is a unique product of the culture that created it. It is also difficult to talk about 'Roman religion' as a whole, since it is really composed of a large number of separate cults: there are many Roman religions. It is a ritualistic religion, a social religion that maintained the well-being of the community <Scheid, p 18-19) a civic religion, the ethical code of the community, the one that prevailed.
The cultus deorum is many layered and includes mystery cults, that catered to the individual as well as state cults and fraternal collegia.
John Scheid, An Introduction to Roman Religion Jorg Rupke, Religion of the Romans
As practised in Nova Roma
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