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This probably deserved more research before posting but I won't be able to do that for a while and it seemed interesting enough. The book covers the subject from the Hindus and Brahmins through the Greeks to freethinkers like Giordano Bruno and Lucilio Vanini to an array of early modern thinkers - Spinoza, Berkeley, Fichte, Hegel, Schopenhauer. If this doesn't seem like purely a religious history you're probably right - Plumptre apparently was an agnostic herself (one book or pamphlet was titled On the Progress of Liberty of Thought During Queen Victoria's Reign and she had an essay in The Agnostic Annual of 1896).
The author appears to be an interesting figure but I've only found sparse information. She was born in England in 1848 and started writing in 1878 when the first volume of the General Sketch appeared anonymously. She used "C.E. Plumptre" for most later publications, the best-known of which was Studies in Little-Known Subjects (which could have been the name of this blog). I found periodical pieces where she didn't support (but also didn't oppose) women's suffrage (Liberty Review, Jan. 1909, p11), was doubtful about taxation (The American Anti-Socialist, v1,n1-6, 1913) and wrote biographical pieces for The Indian Magazine. She died in 1929.
