This is from the "Darwin Correspondence Project". Charles Darwin explained why he didn't waste his time writing about religious bullshit. This was 9 years before he died.

Darwin Correspondence Project

To E. B. Aveling   13 October 1880

Down, | Beckenham, Kent. | (Railway Station | Orpington. S.E.R.)

Oct. 13th 1880

Private

Dear Sir

I am much obliged for your kind letter & the enclosure.— The publication in any form of your remarks on my writings really requires no consent on my part, & it would be ridiculous in me to give consent to what requires none.— I shd. prefer the Part or Volume not to be dedicated to me (though I thank you for the intended honour) as this implies to a certain extent my approval of the general publication, about which I know nothing.— Moreover though I am a strong advocate for free thought on all subjects, yet it appears to me (whether rightly or wrongly) that direct arguments against christianity & theism produce hardly any effect on the public; & freedom of thought is best promoted by the gradual illumination of men’s minds, which follows from the advance of science. It has, therefore, been always my object to avoid writing on religion, & I have confined myself to science. I may, however, have been unduly biassed by the pain which it would give some members of my family, if I aided in any way direct attacks on religion.—

I am sorry to refuse you any request, but I am old & have very little strength, & looking over proof-sheets (as I know by present experience) fatigues me much.—

I remain Dear Sir | Yours faithfully | Ch. Darwin