From Wikipedia I get:
The word religio originally meant an obligation to the gods, something expected by them from human beings or a matter of particular care or concern as related to the gods. In this sense, religio might be translated better as "religious scruple" than with the English word "religion". One definition of religio offered by Cicero is cultus deorum, "the proper performance of rites in veneration of the gods."
Religio among the Romans was not based on "faith," but on knowledge, including and especially correct practice. Religio (plural religiones) was the pious practice of Rome's traditional cults, and was a cornerstone of the mos maiorum, the traditional social norms that regulated public, private, and military life.
Religiosus was something pertaining to the gods or marked out by them as theirs, as distinct from sacer, which was something or someone given to them by men. Hence, a graveyard was not primarily defined as sacer but a locus religiosus, because those who lay within its boundaries were considered belonging to the di Manes. Places struck by lighting were taboo; to man because they had been marked as religiosus by Jupiter himself. See also sacer and sanctus.
Sacer describes a thing or person given to the gods, thus "sacred" to them. Sacer was a fundamental principle in Roman and Italic religions. Anything not sacer was profanum: literally, "in front of (or outside) the shrine", therefore not belonging to it or the gods. A thing or person could be made sacer (consecrated), or could revert from sacer to profanum (deconsecrated), only through lawful rites r(esecratio) performed by a pontiff on behalf of the state. Human beings had no legal or moral claims on anything sacer. A thing not given to the gods but already owned by them or actively marked out by them as divine property was religiosus.
Sanctus, an adjective formed on the past participle of verb sancio, describes that which is "established as inviolable" or "sacred", most times in a sense different to that of sacer and religiosus. In fact its original meaning would be that which is protected by a sanction (sanctio). It is connected to the name of the Umbrian or Sabine founder-deity Sancus (in Umbrian Sancius) whose most noted function was the ratifying and protecting of compacts (foedera). The Roman jurist Ulpian distinguishes sanctus as "neither sacred (sacer) nor profane (profanum) … nor religiosus." Gaius writes that a building dedicated to a god is sacrum, a town's wall and gate are res sanctae because they belong "in some way" to divine law, and a graveyard is religiosus because it is relinquished to the di Manes. Thus some scholars think that it should originally be a concept related to space i.e. concerning inaugurated places, because they enjoyed the armed protection (sanctio) of the gods
Cultus Cicero defined religio as cultus deorum, "the cultivation of the gods." The "cultivation" necessary to maintain a specific deity was that god's cultus, "cult," and required "the knowledge of giving the gods their due" (scientia colendorum deorum). The noun cultus originates from the past participle of the verb colo, colere, colui, cultus, "to tend, take care of, cultivate," originally meaning "to dwell in, inhabit" and thus "to tend, cultivate land (ager); to practice agriculture," an activity fundamental to Roman identity even when Rome as a political center had become fully urbanized. Cultus is often translated as "cult", without the negative connotations the word may have in English, or with the Anglo-Saxon word "worship", but implying the necessity of active maintenance beyond passive adoration. Cultus was expected to matter to the gods as a demonstration of respect, honor, and reverence; it was an aspect of the contractual nature of Roman religion (see do ut des). St. Augustine echoes Cicero's formulation when he declares that "religio is nothing other than the cultus of God."
Fanaticus means "belonging to a fanum," a shrine or sacred precinct. Fanatici as applied to people refers to temple attendants or devotees of a cult, usually one of the ecstatic or orgiastic religions such as that of Cybele (in reference to the Galli), Bellona-Ma, or perhaps Silvanus. Inscriptions indicate that a person making a dedication might label himself fanaticus, in the neutral sense of "devotee". Tacitus uses fanaticus to describe the troop of druids who attended on the Icenian queen Boudica. The word was often used disparagingly by ancient Romans in contrasting these more emotive rites to the highly scripted procedures of public religion, and later by early Christians to deprecate religions other than their own; hence the negative connotation of "fanatic" in English. Festus says that a tree struck by lightning is called fanaticus, a reference to the Romano-Etruscan belief in lightning as a form of divine sign.
Superstitio Excessive devotion and enthusiasm in religious observance, in the sense of "doing or believing more than was necessary." Lucretius's famous condemnation of what is often translated as "Superstition" in his Epicurean didactic epic De rerum natura is actually directed at Religio.
The association of "religion" with gods goes back to the Roman meaning of the word "religio" but not the meaning of religion as a belief system. Basically to the Romans "religio" meant practices not beliefs.
Reading "Meditations" of Marcus Aurelius I've come to think that to Romans of his time "philosophy" is closer to the definition of religion as a system of belief. In Marcus Aurelius' time a philosopher was someone who lived in accord with a set of beliefs, not merely someone who studied ideas.
In "The Christians as the Romans Saw Them" so far, Pliny was mostly concerned with "clubs" or voluntary associations, and superstition. The emperor was concerned that such organizations cause political unrest. There are two kinds of voluntary associations discussed: hetaeria - "clubs" mutual interest societies, and eranus - "benefit societies" formed by poor people for mutual aid. In Pliny's time Christians were rumored to engage in human sacrifice, cannibalism, and orgies (descriptions of supposed Christian practices are quite lurid). There is an irony to Christians being accused of the same things they now accuse others of. Pliny found no evidence of actual crimes, but had them executed for forming illegal "clubs" (hetaeria) and practicing a foreign cult (superstitio). As the Romans believed that "religio" safeguarded the state, both superstitio and atheism threatened the well being of the state, that is the sort of thing you get when you mix government and religion. To prove that they had renounced their cult Pliny asked "ex-Christians" to offer wine and incense to the Roman gods. Those who refused to were executed. He seems to have introduced that particular test of faith.
It is interesting that even at this early stage Christians asked others "only believe, do not ask questions". And there was a lot of turn-over in the Christian cult. Not everyone who joined chose to stay.
I will continue reading and commenting. And I think I need to read "Bowling Alone" next to think some more about the role of "clubs" in society.