Both Uses

Puritans
in
Invisible Man, by Ellison
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Susie Gresham, Mother Gresham, guardian of the hot young women on the puritan benches who couldn't see your Jordan's water for their private steam; you, relic of slavery whom the campus loved but did not understand, aged, of slavery, yet bearer of something warm and vital and all-enduring, of which in that island of shame we were not ashamed-it was to you on the final row I directed my rush of sound, and it was you of whom I thought with shame and regret as I waited for the ceremony to begin.†
Chpt 5 *

Puritan = an English Protestant who in the 16th and 17th centuries who wanted simpler worship and strict, hard-working lives
She was slipping out of her coat now, looking earnestly into my face, and I thought, Is she a Salvationist, a Puritan-with-reverse-English?†
Chpt 19

Definitions:

(1) (Puritans)
English Protestants in the 16th and 17th centuries who wanted simpler, “purer” worship and emphasized hard work over pleasure

Protestants are Christian groups that broke away from the Catholic Church; in the United States, well-known Protestant churches include Baptists, Methodists, Presbyterians, and Episcopalians.
(2) (meaning too rare to warrant focus)
As a common noun (not capitalized unless at the start of a sentence), puritan or puritanical can refer to someone who is very strict -- especially about religious principles or proper behavior.