1799, in reference to the doctrine that all mankind are equal, from equality on model of humanitarian, etc. As a noun from 1837.
Entries linking to equalitarian
equality n.
late 14c., "evenness, smoothness, uniformity;" c. 1400 in reference to amount or number; from Old French equalité "equality, parity" (Modern French égalité, which form dates from 17c.), from Latin aequalitatem (nominative aequalitas) "equality, similarity, likeness" (also sometimes with reference to civil rights), from aequalis "uniform, identical, equal" (see equal (adj.)). Early 15c. as "state of being equal." Of privileges, rights, etc., in English from 1520s.
humanitarian n.
1794 in the theological sense, "one who affirms the humanity of Christ but denies his pre-existence and divinity," from human (adj.) + suffix from unitarian, etc. By 1834 as "one who professes the creed that a person's highest duty is to advance the welfare of the human race," but the closely allied sense "philanthropist, one who advocates or practices human action to solve social problems" (1842), originally was disparaging, with a suggestion of excess. Compare humanism.
As an adjective by 1834 in the theological sense "affirming the humanity or human nature of Christ;" by 1855 as "having regard for the broad interests of humanity."