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.
updated on March 25, 2022