c. 1200, religioun, "state of life bound by monastic vows," also "action or conduct indicating a belief in a divine power and reverence for and desire to please it," from Anglo-French religiun (11c.), Old French religion, relegion "piety, devotion; religious community," and directly from Latin religionem (nominative religio) "respect for what is sacred, reverence for the gods; conscientiousness, sense of right, moral obligation; fear of the gods; divine service, religious observance; a religion, a faith, a mode of worship, cult; sanctity, holiness," in Late Latin "monastic life" (5c.).

This noun of action was derived by Cicero from relegere "go through again" (in reading or in thought), from re- "again" (see re-) + legere "read" (see lecture (n.)). However, popular etymology among the later ancients (Servius, Lactantius, Augustine) and the interpretation of many modern writers connects it with religare "to bind fast" (see rely), via the notion of "place an obligation on," or "bond between humans and gods." In that case, the re- would be intensive. Another possible origin is religiens "careful," opposite of negligens.

In English, the meaning "particular system of faith in the worship of a divine being or beings" is by c. 1300; the sense of "recognition of and allegiance in manner of life (perceived as justly due) to a higher, unseen power or powers" is from 1530s.

His [St. James's] intention is somewhat obscured to the English reader from the fact that 'religious' and 'religion,' by which we have rendered [thrēskos] and [thrēskeia], possessed a meaning once which they now possess no longer, and in that meaning are here employed. St. James is indeed claiming for the new dispensation a superiority over the old, in that its very [thrēskeia] consists in acts of mercy, of love, of holiness, in that it has light for its garment, its very robe being righteousness .... [Archbishop Trench, "Synonyms of the New Testament," 1854]

"lack of religion, contempt of religion, impiety," 1590s, from French irréligion (16c.) or directly from Late Latin irreligionem (nominative irreligio) "irreligion, impiety," from assimiliated form of in- "not" (see in- (1)) + religio (see religion).

It is come, I know not how, to be taken for granted, by many Persons, that Christianity is not so much as a Subject of Inquiry ; but that it is, now at length, discovered to be fictitious. And accordingly they treat it, as if, in the present Age, this were an agreed Point, among all People of Discernment; and nothing remained, but to set it up as a principal Subject of Mirth and Ridicule, as it were by Way of Reprisals, for its having so long interrupted the pleasures of the World. [Joseph Butler, introduction to "The Analogy of Religion," 1740]

c. 1200, "devout, pious, imbued with or expressive of religious devotion," used of Christians, Jews, pagans; also "belonging to a religious order," from Anglo-French religius, Old French religious (12c., Modern French religieux) and directly from Latin religiosus, "pious, devout, reverencing or fearing the gods," also "religiously careful, anxious, or scrupulous," from religio "religious observance; holiness" (see religion).

The meaning "pertaining to religion" is from 1530s. The transferred sense of "scrupulous, exact, conscientious" is recorded from 1590s but restores or revives a sense right at home among the superstitious Romans. As a noun, from c. 1200 as "persons bound by vow to a religious order;" from late 14c. as "pious persons, the devout." Related: Religiousness.

"one addicted to or zealous (often in a bad sense) for a religion," 1650s, from religion + -ist.

word-forming element used from late 19c. with a sense of "religious, pertaining to religion, of religion and," from Latin religio "a religion; holiness" (see religion).

also co-religionist, "one of the same religion as another," 1835, from co- + religion + -ist.

Proto-Indo-European root meaning "to tie, bind." 

It forms all or part of: alloy; ally; colligate; deligate; furl; league (n.1) "alliance;" legato; liable; liaison; lien; lictor; ligand; ligament; ligate; ligation; ligature; oblige; rally (v.1) "bring together;" religion; rely.

It is the hypothetical source of/evidence for its existence is provided by: Latin ligare "to bind;" Albanian lidh "I bind," and possibly Middle Low German lik "band," Middle High German geleich "joint, limb."

c. 1300, "the crime or sin of stealing what is consecrated to God," from Old French sacrilege (12c.), from Latin sacrilegium "temple robbery, a stealing of sacred things," from sacrilegus "temple-robber, stealer of sacred things," noun use of adjective, from phrase sacrum legere "to steal sacred things," from sacrum "sacred object" (from neuter singular of sacer "sacred;" see sacred) + legere "take, pick up" (from PIE root *leg- (1) "to collect, gather").

The second element is not from religion, and the two words might not be related etymologically. From early 14c. as "improper or impious behavior." The transferred sense of "profanation of anything held sacred" is attested from late 14c.

mid-15c., "adherent of a religion opposed to Christianity," from Old French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (from PIE root *bheidh- "to trust, confide, persuade").

Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see Kaffir), which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."

"lukewarm in religion," 1560s, from Laodicea, ancient city of Phrygia Minor (modern Latakia in Syria) whose early Christians were chastised in the Bible for indifference to their religion (Revelation iii:14-16). The city is said to be named for the 3c B.C.E. Syrian queen Laodice, wife of Antiochus II.