Sir Arthur C. Clarke was a British science fiction author and futurist, best known for the novel 2001: A Space Odyssey and as a host of the British television series Mysterious World. For many years, he was considered part of the “Big Three” of science fiction, along with Isaac Asimov and Robert A. Heinlein. 
The following is a passage from his biography, Arthur C. Clarke: The Authorized Biography (pp. 311), by Neil McAleer:
Lee Lubbers arrived at Barnes Place one day in September 1983, ahead of the main contingent. Upon his arrival he was escorted into Clarke's study.
"He stepped out from behind his desk," says Lubbers, "and dashed across the room with his right hand extended to shake hands, all the while saying (as though he were protesting dramatically that I was going to convert him before he could reach the other end of the room), 'I am an atheist.' Well, you can define atheist any way you want. I'm not sure that he even meant he was an atheist. Most of us don't really know what we're talking about, unless we profess to belong to some camp. It might have been an invitation to dialogue. It certainly was an invitation to friendship, I think. I look upon his anticlerical stance as being very friendly and as coming from a person who is very secure about himself and his own image and can afford to do things like that. 
See his autobiography. Also, in The Making of Kubrick's 2001, Clarke says that he is an atheist.
It may be that our role on this planet is not to worship God, but to create him. -- Arthur C. Clarke
In a 1988 talk show entitled "God, the Universe, and Everything Else" interviewer Magnus Magnusson asks Clarke what he meant when he was alleged to have said to the Papal Nuncio "I don't believe in God but I'm extremely interested in him." Clarke replies, "Well, I guess I hadn't placed my bets yet," and then goes on to say, "200 years ago, when Napoleon was talking to Laplace, who published his theory of the universe, Napoleon said 'Well, God wasn't in it,' Laplace replied, 'Sir, I have no need for that hypothesis.'"
In an April 1, 1997 profile in the New York Times Clarke speaks about his new book 3001, the latest and perhaps final in the series of books beginning with 2001:
In the world of 3001 Clarke envisions for the story, the writer of the piece, John F. Burns, says: "Perhaps most controversially, religions of all kinds have fallen under a strict taboo, with the citizenry looking back on the religious beliefs and practices of earlier ages as products of ignorance that caused untold strife and bloodshed. But the concept of a God, known by the Latin word Deus, survives, a legacy of man's continuing wonder at the universe.
"In this, Clarke is giving vent to one of the few things that seem to ruffle his equable nature. 'Religion is a byproduct of fear,' he says. 'For much of human history, it may have been a necessary evil, but why was it more evil than necessary? Isn't killing people in the name of God a pretty good definition of insanity?' "
28 May 2001 - A reader reports that in a CNN interview when Clarke was asked if he believed in God, he replied, "I do not believe in God, but I do not disbelieve in her either." If anybody can confirm this and provide a date of the interview, please send it in. Thanks.
A reader writes: Arthur C. Clarke was a strong atheist. When he was in the British military, he was adamant about having his religion listed as 'atheist'. Refer to his biography for more info.
Clarke reportedly says he is an atheist in a 20-Dec-2000 interview in The Island, a Sri Lankan newspaper. http://www.island.lk/2000/12/20/midwee01.html