Etymology
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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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sect (n.)

mid-14c., "a distinctive system of beliefs or observances held by a number of persons; a party or school within a religion," from Old French secte, sete "sect, religious community" (14c.) and directly from Late Latin secta "religious group, sect in philosophy or religion," especially a heretical one. This is a special development of Latin secta "manner, mode; following; school of thought; course, system," literally "a way, a road, a beaten path," from fem. of sectus, variant past participle of sequi "follow," from PIE root *sekw- (1) "to follow." General sense of "those of a certain way of thinking or living" is from late 14c.

The notion in the Late Latin development is "those following (someone's) way." But the history of the word seems to be confused with that of Latin secta, fem. past participle of secare "to cut" (from PIE root *sek- "to cut"). The meaning "separately organized religious body, denomination" is recorded from 1570s in a Protestant context and seems to carry more of a notion of a party "cut off" from a main body.

It also was used in Middle English generally of a class of people or things, a species or race, a distinctive costume, sometimes also of sex (perhaps partly by confusion with that word).

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sectary (n.)

"member or adherent of a sect," 1550s, from French sectaire or directly from Medieval Latin sectarius, from secta "religious group, sect in philosophy or religion" (see sect).

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non-sectarian (adj.)

also nonsectarian, "not involving or relating to a specific religious sect," 1825, from non- + sectarian.

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sectarian (adj.)

1640s, "belonging or pertaining to a schismatic sect," applied by Presbyterians to Independents, from Medieval Latin sectarius, from secta "religious group, sect in philosophy or religion" (see sect). By 1796 as "of or pertaining to sects or to attachment to a particular sect; including the tenets of a sect," hence "bigotedly attached to a sect." Sectarial (1816) is "Chiefly used with reference to Indian religions" [OED].

As a noun, "one of a sect" (1650s), especially "one who attaches excessive importance to a sect." The older word in this sense is sectary.

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denominational (adj.)

"pertaining to a religious denomination or sect," 1838; see denomination + -al (1). Related: Denominationalism "tendency to divide into sects or denominations, inclination to emphasize distinguishing characteristics as opposed to general principles" (1845).

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seeker (n.)

c. 1300, secher (late 13c. as a surname), "one who searches, investigator," agent noun from seek. The religious sect of the Seekers is attested from 1645; they professed no set religion but said they went in search of a true ministry.

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heresy (n.)

"doctrine or opinion at variance with established standards" (or, as Johnson defines it, "an opinion of private men different from that of the catholick and orthodox church"), c. 1200, from Old French heresie, eresie "heresy," and by extension "sodomy, immorality" (12c.), from Latin hæresis, "school of thought, philosophical sect." The Latin word is from Greek hairesis "a taking or choosing for oneself, a choice, a means of taking; a deliberate plan, purpose; philosophical sect, school," from haireisthai "take, seize," middle voice of hairein "to choose," a word of unknown origin, perhaps cognate with Hittite šaru "booty," Welsh herw "booty;" but Beekes offers "no etymology."

The Greek word was used by Church writers in reference to various sects, schools, etc. in the New Testament: the Sadducees, the Pharisees, and even the Christians, as sects of Judaism. Hence the meaning "unorthodox religious sect or doctrine" in the Latin word as used by Christian writers in Late Latin. But in English bibles it usually is translated sect. Transferred (non-religious) use in English is from late 14c.

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set (n.1)
"collection of things," mid-15c., from Old French sette "sequence," variant of secte "religious community," from Medieval Latin secta "retinue," from Latin secta "a following" (see sect). "[I]n subsequent developments of meaning influenced by SET v.1 and apprehended as equivalent to 'number set together'" [OED]. The noun set was in Middle English, but only in the sense of "religious sect" (late 14c.), which likely is the direct source of some modern meanings, such as "group of persons with shared status, habits, etc." (1680s).

Meaning "complete collection of pieces" is from 1680s. Meaning "group of pieces musicians perform at a club during 45 minutes" (more or less) is from c. 1925, though it is found in a similar sense in 1580s. Set piece is from 1846 as "grouping of people in a work of visual art;" from 1932 in reference to literary works.
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Sabian (n.)

an adherent of a religious sect mentioned thrice in the Qu'ran (in which they are classified with Christians and Jews as monotheistic "true believers" tolerated by Muslims), 1610s, from Arabic, but a word of uncertain origin. As an adjective from 1748.

Perhaps the reference in the word is to a Gnostic sect akin to the later Mandæans (if the word derives, as some linguists think it does, from Arabic ch'bae "to baptize," Aramaic tzebha "he dipped, dyed"); but it has the appearance of derivation from the Semitic root of Hebrew tzabha "host, army" (see Sabaoth), and as the Sabians were thought to have been star-worshippers, the word was interpreted as referring to the "host of heaven." Related: Sabaism "star-worship" (Century Dictionary says Sabeanism is incorrect).

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