Millions upon millions were sacrificed upon the altars of bigotry. The Catholic burned the Lutheran, the Lutheran burned the Catholic, the Episcopalian tortured the Presbyterian, the Presbyterian tortured the Episcopalian. Every denomination killed all it could of every other; and each Christian felt in duty bound to exterminate every other Christian who denied the smallest fraction of his creed.

Robert Green Ingersoll (1833-1899) American lawyer, agnostic, orator

Lecture (1874-05-03), “Heretics and Heresies,” Free Religious Society, Kingsbury Hall, Chicago
    (Source)

Collected in The Gods and Other Lectures (1876).