From Lewis and Short's Dictionary, s.v. religio: "Cicero derives it from relegere, an opinion favoured by the verse, religentem esse oportet, religiosum nefas: whereas Servius, Lactantius, Augustine, assume religare as the primitive, and for this derivation Lactantius cites the expression of Lucretius, religionum nodis animos exsolvere. Modern etymologists mostly agree with this latter view, assuming as root lig (to bind), whence also lex: hence religio sometimes means the same as obligatio."

Of God and His Creatures: 3.119