This collection of letters explores Darwin’s reluctance to take a definitive position on the nature of God through correspondence with a variety of people, including members of his own family.
Letter 441 — Wedgwood, Emma to Darwin, C. R., [21–22 Nov 1838]
In this letter, his soon-to-be wife, Emma, writes lovingly to Darwin of small events since he left Maer. She fears their opinions may differ on “the most important subject”, religion, but is grateful for his openness about his “honest & conscientious doubts”.
Letter 471 — Darwin, Emma to Darwin, C. R., [c. Feb 1839]
Emma discusses Darwin’s religious doubts. She fears his work may lead him to discount what cannot be proved, and advises that there are some things which, “if true are likely to be above our comprehension” and “that there is a danger in giving up revelation”.
Letter 2534 — Kingsley, Charles to Darwin, C. R., 18 Nov 1859
Clergyman Charles Kingsley judges Darwin’s book [Origin] free from two superstitions: the dogma of the permanent species and the need of an act of intervention to bring change.
Letter 2548 — Sedgwick, Adam to Darwin, C. R., 24 Nov 1859
Woodwardian Professor of geology, Adam Sedgwick thanks Darwin for the Origin; but has read the book “with more pain than pleasure”. He talks about his dislike of the conclusion of the book and how he instead “humbly accepts God’s revelation of himself both in His works & in His word; & will do my best to act in conformity with that knowledge which only He can give me.”
Letter 5303 — Boole, M. E. to Darwin, C. R., 13 Dec 1866
In this letter marked “private”, mathematician and teacher Mary Boole asks whether Darwin believes natural selection obviates man’s ability to be guided by spiritual motives. She is anxious that his theory be compatible with her faith.
Letter 5307 — Darwin, C. R. to Boole, M. E., 14 Dec 1866
Darwin believes he is unable to answer Mary Boole’s questions about religious implications of natural selection, but would prefer to believe that suffering in world is due to natural events. In the postscript, he feels that theology and science should each run its own course.
Letter 8070 — Darwin, C. R. to Abbot, F. E., 16 Nov 
Darwin explains why he must decline to write for the Index: his health is poor and he has never systematically thought much on religion. He says Abbot may print his comments, “with qualifications”, if he wishes.
Letter 8837 — Darwin, C. R. to Doedes, N. D., 2 Apr 1873
Darwin explains the impossibility of conceiving that the universe arose through chance is the chief argument for the existence of God, but Darwin has never been able to decide whether this is an argument of real value. He thinks it is safest to believe that the subject is beyond man’s intellect, “but man can do his duty”.
Letter 12041 — Darwin, C. R. to Fordyce, John, 7 May 1879
In this letter marked “private”, Darwin says to John Fordyce that he believes it is absurd to doubt that a man may be an ardent theist and evolutionist, giving the examples of Kingsley and Asa Gray. As regards his own views, his judgement often fluctuates but “I have never been an Atheist in the sense of denying the existence of God”. He thinks that “generally (and more and more as I grow older) … an Agnostic would be the most correct description of my state of mind”.
Letter 12757 — Darwin, C. R. to Aveling, E. B., 13 Oct 1880
In this letter marked “private”, Darwin says to Physician E. B. Aveling that the publication of Aveling’s remarks on his writings requires no consent on his part. Darwin would prefer that no part or volume be dedicated to him as it would imply his general approval of the publication, of which he knows nothing. Though he is a strong advocate of free-thought, Darwin feels that direct attacks on Christianity and theism produce hardly any effect. Freedom of thought is best promoted by gradual illumination of men’s minds produced by advance of science. He has therefore avoided writing on religion but states “I may have been unduly biassed by the pain which it would give members of my family, if I aided in any way direct attacks on religion”.