Etymology
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believer (n.)
1540s, "one who has faith in religion," agent noun from believe. From c. 1600 as "one who gives credence (to anything) without personal knowledge, one firmly persuaded of the truth of something."
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*were-o- 
*wērə-o-, Proto-Indo-European root meaning "true, trustworthy."

It forms all or part of: aver; Varangian; veracious; veracity; verdict; veridical; verify; verisimilitude; verism; veritas; verity; very; voir dire; warlock.

It is the hypothetical source of/evidence for its existence is provided by: Latin verus "true;" Old Church Slavonic vera "faith," Russian viera "faith, belief;" Old English wær "a compact," Old Dutch, Old High German war, Dutch waar, German wahr "true;" Welsh gwyr, Old Irish fir "true."
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perfidy (n.)

"breach of faith or trust, base treachery," 1590s, from French perfidie (16c.), from Latin perfidia "faithlessness, falsehood, treachery," from perfidus "faithless," from phrase per fidem decipere "to deceive through trustingness," from per "through" (from PIE root *per- (1) "forward," hence "through") + fidem (nominative fides) "faith" (from PIE root *bheidh- "to trust, confide, persuade").

[C]ombinations of wickedness would overwhelm the world by the advantage which licentious principles afford, did not those who have long practiced perfidy grow faithless to each other. [Samuel Johnson, "Life of Waller"]
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trig (adj.)
"smart, trim," c. 1200, from Old Norse tryggr "firm, trusty, true," from Proto-Germanic *treuwaz "having or characterized by good faith," from PIE *drew-o-, a suffixed form of the root *deru- "be firm, solid, steadfast." A Scottish and northern word only until 19c. Related: Trigness.
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convert (n.)

1560s, "person whose faith has been changed from one religion to another," from convert (v.). Earlier was convers (early 14c.), from Old French converse (n.). General (non-religious) sense of "person converted from one opinion or practice to another" is from 1640s.

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Novatian (n.)

mid-15c., member of an early Christian sect founded mid-3c. by the theologian Novatianus (c. 200-258). The schism involved readmission of Christians who had denied their faith under the Decian persecution (Novatianus favored strict treatment and non-forgiveness). Related: Novatianism.

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piety (n.)

mid-14c., piete (late 12c. as a surname), "mercy, tenderness, pity" (senses now obsolete in this word but preserved in its doublet, pity), from Old French piete "piety, faith; pity, compassion" (12c.), from Latin pietatem (nominative pietas) "dutiful conduct, sense of duty; religiousness, piety; loyalty, patriotism; faithfulness to natural ties," in Late Latin "gentleness, kindness, pity;" from pius "kind" (see pious).

From 1570s in English as "filial affection, dutiful conduct or behavior toward one's parents, relatives, country, etc." Meaning "piousness, faith in and reverence for the Supreme Being" is attested in English from c. 1600. Compare pity (n.).

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Judaism (n.)
c. 1400 (attested in Anglo-Latin from mid-13c.), from Old French Judaisme and directly from Late Latin Judaismus, from Greek Ioudaismos, from Ioudaios "Jew" (see Jew). The Anglo-Latin reference is from a special tax levied on the Jews of England. Earlier in same sense was Juhede "Jewish faith, Judaism," literally "Jew-hood" (early 14c.).
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baptist (n.)
c. 1200, "one who baptizes," also (with capital B-) a title of John, the forerunner of Christ; see baptize + -ist. As "member of a Protestant sect that believes in adult baptism upon profession of faith," generally by full immersion (with capital B-), attested from 1654; their opponents called them anabaptists (see Anabaptist).
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superstitious (adj.)
late 14c., "involving faith in supernatural powers or magic; characteristic of pagan religion or false religion," from Anglo-French supersticius, Old French supersticios, or directly from Latin superstitiosus "prophetic; full of dread of the supernatural," from superstitio "prophecy, soothsaying, excessive fear of the gods" (see superstition).
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