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 and theism produce hardly any effect on the public; and freedom of thought is best promoted by 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, and I have confined myself to science. I may, however, have been unduly biased by the pain which it would give some members of my family, if I aided in any way direct attacks on religion.
From letter to Edward Aveling