Howard Phillips Lovecraft (1890-1937) was a writer of fantasy and horror fiction, and the inventor of the Cthulhu mythos.
He asserts his atheism in the essay "A Confession of Unfaith", which is reprinted in the collection Miscellaneous Writings edited by S. T. Joshi.
(Second Opinion: I disagree. I read "A Confession of Unfaith" to vet his supposed assertion of atheism and found no such quote. Rather, the essay detailed his long and precocious religious/spiritual evolution, which led him to a tough-to-pigeonhole belief which perhaps is best categorized as Pagan. His views are MASSIVELY complex. So much so that I found it impossible to simply post a neat quote that sums up his beliefs. I'd really have to type up the full 6 or 8 page essay....which I'm not that keen on doing.) ---
"If religion were true, its followers would not try to bludgeon their young into an artificial conformity; but would merely insist on their unbending quest for truth, irrespective of artificial backgrounds or practical consequences." -- need source