Today, Religion is commonly used as a social genus. However, the word religion did not originally refer to a group. religion originally applied to individuals. The concept shifted from an individual to a group context through a historical process that took many years.
Even in ancient times, people were aware that other groups of people worshipped other gods and had practices related to that worship, seeing them as rivals. In this context, the word religio was used with nobis. Nobis religio meant “our way to worship.” This is the closest use of the word religio in relation with a group in ancient times. Christians would later use the word religio to explicitly refer to a group of people following specific rules. They used religio (religiones plural) to refer to a monastic order.
The fact that religio was used in both divine and human contexts was the reason why Augustine rejected the word religio to refer to “how one worships God.” As centuries passed by, the meaning of the word religion has been incoherent and The incoherence of the word religion questions its essence and realness.
Schilbrack, K. (2022). The Concept of Religion. In E. N. Zalta (Ed.), The Stanford Encyclopedia of Philosophy (Summer 2022). Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/archives/sum2022/entries/concept-religion/
In western antiquity, and likely in many or most cultures, there was a recognition that some people worshipped different gods with commitments that were incompatible with each other and that these people constituted social groups that could be rivals. In that context, one sometimes sees the use of nobis religio to mean “our way of worship”.
Nevertheless, religio had a range of senses and so Augustine could consider but reject it as the right abstract term for “how one worships God” because the Latin term (like the Latin terms for “cult” and “service”) was used for the observance of duties in both one’s divine and one’s human relationships
In the Middle Ages, as Christians developed monastic orders in which one took vows to live under a specific rule, they called such an order religio (and religiones for the plural)