Etymology
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religion (n.)
Origin and meaning of religion

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.

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religionless (adj.)

"lacking or having lost religion," 1707, from religion + -less.

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religionist (n.)

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

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religio- 

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).

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coreligionist (n.)

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

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religious (adj.)

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.

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irreligion (n.)

"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]
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sacrilege (n.)

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.

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*leig- 

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."

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re- 

word-forming element meaning "back, back from, back to the original place;" also "again, anew, once more," also conveying the notion of "undoing" or "backward," etc. (see sense evolution below), c. 1200, from Old French re- and directly from Latin re- an inseparable prefix meaning "again; back; anew, against."

Watkins (2000) describes this as a "Latin combining form conceivably from Indo-European *wret-, metathetical variant of *wert- "to turn." De Vaan says the "only acceptable etymology" for it is a 2004 explanation which reconstructs a root in PIE *ure "back."

In earliest Latin the prefix became red- before vowels and h-, a form preserved in redact, redeem, redolent, redundant, redintegrate, and, in disguise, render (v.). In some English words from French and Italian re- appears as ra- and the  following consonant is often doubled (see rally (v.1)).

The many meanings in the notion of "back" give re- its broad sense-range: "a turning back; opposition; restoration to a former state; "transition to an opposite state." From the extended senses in "again," re- becomes "repetition of an action," and in this sense it is extremely common as a formative element in English, applicable to any verb. OED writes that it is "impossible to attempt a complete record of all the forms resulting from its use," and adds that "The number of these is practically infinite ...."   

Often merely intensive, and in many of the older borrowings from French and Latin the precise sense of re- is forgotten, lost in secondary senses, or weakened beyond recognition, so that it has no apparent semantic content (receive, recommend, recover, reduce, recreate, refer, religion, remain, request, require). There seem to have been more such words in Middle English than after, e.g. recomfort (v.) "to comfort, console; encourage;" recourse (n.) "a process, way, course." Recover in Middle English also could mean "obtain, win" (happiness, a kingdom, etc.) with no notion of getting something back, also "gain the upper hand, overcome; arrive at;" also consider the legal sense of recovery as "obtain (property) by judgment or legal proceedings." 

And, due to sound changes and accent shifts, re- sometimes entirely loses its identity as a prefix (rebel, relic, remnant, restive, rest (n.2) "remainder," rally (v.1) "bring together"). In a few words it is reduced to r-, as in ransom (a doublet of redemption), rampart, etc.

It was used from Middle English in forming words from Germanic as well as Latin elements (rebuild, refill, reset, rewrite), and was used so even in Old French (regret, regard, reward, etc.).

Prefixed to a word beginning with e, re- is separated by a hyphen, as re-establish, re-estate, re-edify, etc. ; or else the second e has a dieresis over it: as, reëstablish, reëmbark, etc. The hyphen is also sometimes used to bring out emphatically the sense of repetition or iteration : as, sung and re-sung. The dieresis is not used over other vowels than e when re is prefixed : thus, reinforce, reunite, reabolish. [Century Dictionary, 1895]
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