Mr. Spong is a retired Episcopal Bishop. He has written books on religion such as Rescuing the Bible From Fundamentalism, Born of a Woman, The Resurrection: Myth or Reality, Why Christianity Must Change or Die, and A New Christianity For A New World.
Mr. Spong may be an odd choice to add to your website because he still calls himself a Christian and still claims to believe in God.
However, when you read Why Christianity Must Change or Die and A New Christianity For A New World, he really is an atheist.
He doesn't believe God is a supernatural or transcendental being. He doesn't believe in a personal God. He rejects theism. He calls himself a nontheist. However, he says nontheism is not atheism.
Here is his definition of theism which he rejects: "I define the theistic God as a 'being supernatural in power, dwelling outside this world and invading the world periodically to accomplish the divine will" (p. 61 A New...World).
"The evidence that God, understood theistically is dying or is perhaps already dead is overwhelming" (p. 21 A New...World).