Robert Anson MacDonald Heinlein (1907-1988) was a science-fiction writer.
He is usually considered an atheist, though I have not been able to find direct assertions to that effect. However, he has made numerous anti-religious comments, like
Men rarely (if ever) managed to dream up a god superior to themselves. Most gods have the manners and morals of a spoiled child. -- Robert A. Heinlein, quoted in McWilliams, Ain't Nobody's Business If You Do, p. 375, quoted from James A. Haught, ed., 2000 Years of Disbelief
The faith in which I was brought up assured me that I was better than other people; I was saved, they were damned.... Our hymns were loaded with arrogance -- self-congratulation on how cozy we were with the Almighty and what a high opinion he had of us, what hell everybody else would catch come Judgment Day. -- Robert A. Heinlein, from Laurence J. Peter, Peter's Quotations: Ideas for Our Time, quoted from James A. Haught, ed., 2000 Years of Disbelief, quoted from James A. Haught, ed., 2000 Years of Disbelief