Etymology
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sect (n.)
mid-14c., "distinctive system of beliefs or observances; party or school within a religion," from Old French secte, sete "sect, religious community," or directly from Late Latin secta "religious group, sect in philosophy or religion," from Latin secta "manner, mode, following, school of thought," literally "a way, road, beaten path," from fem. of sectus, variant past participle of sequi "follow," from PIE root *sekw- (1) "to follow." Confused in this sense with Latin secta, fem. past participle of secare "to cut" (from PIE root *sek- "to cut"). Meaning "separately organized religious body" is recorded from 1570s.
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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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non-sectarian (adj.)

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

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seeker (n.)
early 14c., agent noun from seek. The religious sect of the Seekers is attested from 1645.
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Lutheran 
1520s, adjective and noun, "of or pertaining to Martin Luther or to the sect he founded, which has his name, or its doctrines," from name of German religious reformer Martin Luther (1483-1546). Luther called it the Evangelical Church. Used by Catholics 16c. in reference to all Protestants, regardless of sect. In 16c. Lutherian also was used. Related: Lutheranism (1560s).
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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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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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sectary (n.)
"member or adherent of a sect," 1550s, from French sectaire or directly from Medieval Latin sectarius, from secta (see sect).
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Druse 

also Druze, one of a people and Muslim sect centered in the mountains of Lebanon, 1786, from Arabic duruz, plural of darazi, from name of the sect founder, Ismail ad-Darazi (11c.), literally "Ismail the Tailor."

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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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