"[Darwin] avoided discussing his views on religion in public, but his private writings (from the Beagle era onwards) show that he was increasingly critical of theology and Biblical literalism, and had many doubts about God, including concerns about the problem of evil. In a letter of 1879, he described himself as an #agnostic rather than an atheist, but stated that belief in evolution is not incompatible with #theism."
Citation: Letter to John Fordyce, 7 May 1879, transcribed at the Darwin Correspondence Project