religio

Religio, religionis, f., is third declension, and the classical sense is not the modern one. It is scruple: the binding sense that a rite must be performed exactly, the awe behind it, and by extension the observances themselves. A Roman did not belong to a religio the way one belongs to a church.

Cicero derives it from relegere, "go over again, be careful"; the Christian writers preferred religare, "bind fast". Both are guesses and the word is older than either etymology. Two things to watch: religio can mean superstitious dread, and religio est with an infinitive means "it is forbidden by scruple", not "it is religious". In medieval Latin a religiosus is specifically a monk or a nun, someone under a rule.