John Scheid, "Graeco Ritu: A Typically Roman Way of Honoring the Gods," Harvard Studies in Classical Philology
97 (1995) 15-31 (at 17-18):
To our way of thinking, a ritual is a religious—or at least public—celebration. As a traditional set of gestures and behavior, it is currently opposed and subordinated to the interior and spiritual approach to the divine. I shall not go into details, but this definition of ritual and consequently of ritus is a construction of modern times whose origin reaches back, let us say, to the Reformation. Anyway, this was not the precise meaning of ritus. This did not define the content of a divine service, but only the general custom, the rule followed in celebrating this service. Ritus is not equivalent to sacra, caerimoniae, or religiones, but to mos, the way of doing something, the τρόπος or the νόμος. You have the mos of birds, of horses, of human beings or of Romans, Albans, Greeks, barbarians, and so on. In the religious field, the difference did not lie in the content of the celebration itself, but in the way of celebrating the ceremonies, whatever they were. The meaning of ritus is to be referred to the notion which the ancients had of religion. There was not a true and a false religion. National religion was not radically different from foreign religions. Even the religions of the barbarians were not substantially different from the religions of civilised people. Everywhere people made sacrifices, prayers, and vows, celebrated sacred games, and built sanctuaries. The same terminology was used for the description of all these celebrations, not to mention the net of interpretation which connected the gods of the oikoumene. But one thing made the difference between the religions of the world: the governing rules, those small details, choices, and postures which gave each system its originality, on occasion its perversion. Some individuals or people were qualified as superstitious, not because they venerated the wrong gods or celebrated ridiculous ceremonies, but because they performed their cult in the wrong way; this means for example that they did it in an excessively fearful way, an attitude which did not accord with the dignitas of a god or a citizen. The Celts of Gaul were classed as barbarians, not because they adored idols or celebrated sacrifices, but because their rules did not restrain them from sacrificing human victims: the ritus of these barbarians consisted in offering human beings, not in the other proceedings of the sacrifice. In short, the ritus was the special posture and prescription which gave all public celebrations a special, recognizable tonality—I would compare it to the musical modes: you had the ritus of the Romans, the ritus of the Greeks, the ritus of the barbarians, and so on.