The etymology of the word "religion" has been debated for centuries. The English word clearly derives from the Latin religio, "reverence (for the gods)" or "conscientiousness". The origins of religio, however, are obscure. Proposed etymological interpretations include:
Re-reading–from Latin re (again) + lego (in the sense of "read"), referring to the repetition of scripture. Treating carefully–from Latin re (again) + lego (in the sense of "choose"–this was the interpretation of Cicero) "go over again" or "consider carefully".
Re-connection to the divine–from Latin re (again) + ligare (to connect, as in English ligament). This interpretation is favoured by modern scholars such as Tom Harpur, but was made prominent by St. Augustine, following the interpretation of Lactantius. To bind or return to bondage–an alternate interpretation of the "reconnection" etymology emphasizing a sense of servitude to God, this may have originated with Augustine. However, the interpretation, while popular with critics of religion, is often considered imprecise and possibly offensive to followers.
From Res + legere
Concerning a gathering — from Latin res (ablative re, with regard to) + legere (to gather), since organized religion revolves around a gathering of people.