“The chiliasts made an atheist out of me.”

Source: The Blind Watchmaker (1986), Chapter 1 “Explaining the Very Improbable” (p. 6)

Source: Minimal Art: A Critical Anthology', 1995, p. 107

As quoted in What Great Men Think of Religion (1945) by Ira D Cardiff

http://movies.nytimes.com/2010/02/11/movies/11radical.html
Other sourced statements

In his letter to Theo, from Etten, c. 21 December 1881, Letter #164 http://webexhibits.org/vangogh/letter/10/164.htm, as translated by Mrs. Johanna van Gogh-Bonger, as published in The Complete Letters of Vincent van Gogh (1991) edited by Robert Harrison] <!-- also quoted in Dear Theo: The Autobiography of Vincent Van Gogh (1995) Edited by Irving Stone -->
1880s, 1881
Context: That God of the clergymen, He is for me as dead as a doornail. But am I an atheist for all that? The clergymen consider me as such — be it so; but I love, and how could I feel love if I did not live, and if others did not live, and then, if we live, there is something mysterious in that. Now call that God, or human nature or whatever you like, but there is something which I cannot define systematically, though it is very much alive and very real, and see, that is God, or as good as God. To believe in God for me is to feel that there is a God, not a dead one, or a stuffed one, but a living one, who with irresistible force urges us toward aimer encore; that is my opinion.

Vorkosigan Saga, Shards of Honor (1986)

Attributed by photo meme to an appearance on The Phil Donahue Show in 1989. According to Snopes.com http://www.snopes.com/trump-donahue-atheist-1989/, there is no evidence he ever said this, nor that he even appeared on The Phil Donahue Show in 1989.
Misattributed

The Philosophy of Paine (1925)
Context: He has been called an atheist, but atheist he was not. Paine believed in a supreme intelligence, as representing the idea which other men often express by the name of deity.
His Bible was the open face of nature, the broad skies, the green hills. He disbelieved the ancient myths and miracles taught by established creeds. But the attacks on those creeds — or on persons devoted to them — have served to darken his memory, casting a shadow across the closing years of his life.
When Theodore Roosevelt termed Tom Paine a "dirty little atheist" he surely spoke from lack of understanding. It was a stricture, an inaccurate charge of the sort that has dimmed the greatness of this eminent American. But the true measure of his stature will yet be appreciated. The torch which he handed on will not be extinguished.

Books, Brain Droppings (1997)