More appropriately, what kind of God did Einstein believe in? A new biography of Einstein has just come out. Its called Einstein: His Life and Universe by Walter Isaacson. You can read an excerpt from the book about Einstein's faith here.
About his definition of God:
Einstein tried to express these feelings clearly, both for himself and all of those who wanted a simple answer from him about his faith. So in the summer of 1930, amid his sailing and ruminations in Caputh, he composed a credo, "What I Believe," that he recorded for a human-rights group and later published. It concluded with an explanation of what he meant when he called himself religious: "The most beautiful emotion we can experience is the mysterious. It is the fundamental emotion that stands at the cradle of all true art and science. He to whom this emotion is a stranger, who can no longer wonder and stand rapt in awe, is as good as dead, a snuffed-out candle. To sense that behind anything that can be experienced there is something that our minds cannot grasp, whose beauty and sublimity reaches us only indirectly: this is religiousness. In this sense, and in this sense only, I am a devoutly religious man."Einstein on atheism:
People found the piece evocative, and it was reprinted repeatedly in a variety of translations. But not surprisingly, it did not satisfy those who wanted a simple answer to the question of whether or not he believed in God. "The outcome of this doubt and befogged speculation about time and space is a cloak beneath which hides the ghastly apparition of atheism," Boston's Cardinal William Henry O'Connell said. This public blast from a Cardinal prompted the noted Orthodox Jewish leader in New York, Rabbi Herbert S. Goldstein, to send a very direct telegram: "Do you believe in God? Stop. Answer paid. 50 words." Einstein used only about half his allotted number of words. It became the most famous version of an answer he gave often: "I believe in Spinoza's God, who reveals himself in the lawful harmony of all that exists, but not in a God who concerns himself with the fate and the doings of mankind."
But throughout his life, Einstein was consistent in rejecting the charge that he was an atheist. "There are people who say there is no God," he told a friend. "But what makes me really angry is that they quote me for support of such views." And unlike Sigmund Freud or Bertrand Russell or George Bernard Shaw, Einstein never felt the urge to denigrate those who believed in God; instead, he tended to denigrate atheists. "What separates me from most so-called atheists is a feeling of utter humility toward the unattainable secrets of the harmony of the cosmos," he explained.On the relationship between science & religion:
In fact, Einstein tended to be more critical of debunkers, who seemed to lack humility or a sense of awe, than of the faithful. "The fanatical atheists," he wrote in a letter, "are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who--in their grudge against traditional religion as the 'opium of the masses'-- cannot hear the music of the spheres."
Einstein later explained his view of the relationship between science and religion at a conference at the Union Theological Seminary in New York. The realm of science, he said, was to ascertain what was the case, but not evaluate human thoughts and actions about what should be the case. Religion had the reverse mandate. Yet the endeavors worked together at times. "Science can be created only by those who are thoroughly imbued with the aspiration toward truth and understanding," he said. "This source of feeling, however, springs from the sphere of religion." The talk got front-page news coverage, and his pithy conclusion became famous. "The situation may be expressed by an image: science without religion is lame, religion without science is blind."But he was very clear about his deistic views:
But there was one religious concept, Einstein went on to say, that science could not accept: a deity who could meddle at whim in the events of his creation. "The main source of the present-day conflicts between the spheres of religion and of science lies in this concept of a personal God," he argued. Scientists aim to uncover the immutable laws that govern reality, and in doing so they must reject the notion that divine will, or for that matter human will, plays a role that would violate this cosmic causality.Also check out Walter Isaacson's interview on Fresh Air about this new biography.