A handwritten letter in which Albert Einstein questioned the existence of God is going up for sale on eBay with bids starting at $3 million, just four years after it fetched $404,000 at auction.

The so-called "God letter," which the brilliant physicist wrote in German a year before his death in 1955, includes his thoughts on religion, God and tribalism. Einstein wrote the letter on Princeton University letterhead to philosopher Erik Gutkind after he read Gutkind's book "Choose Life: The Biblical Call to Revolt."

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"...The word God is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honorable, but still primitive legends which are nevertheless pretty childish. No interpretation, no matter how subtle, can (for me) change this," wrote Einstein, who won the Nobel Prize in Physics in 1921.

"For me the Jewish religion like all other religions is an incarnation of the most childish superstitions," the letter continues. "And the Jewish people to whom I gladly belong and with whose mentality I have a deep affinity have no different quality for me than all other people. As far as my experience goes, they are also no better than other human groups, although they are protected from the worst cancers by a lack of power. Otherwise I cannot see anything 'chosen' about them."

The letter ends, "With friendly thanks and best wishes, Yours, A. Einstein."

The letter has been stored in a temperature-controlled vault since it was last sold for $404,000 -- which was 25 times the pre-auction estimate. The buyer at the time is now selling it with the original envelope, stamp and postmark. Eric Gazin, president of Los Angeles-based Auction Cause, which will handle the sale on eBay, said the letter could fetch as much as triple the opening bid threshold. The bidding runs from Oct. 8-18, and information can be seen at www.einsteinletter.com.

"Few people have had access to the thoughts and uncensored opinions of this brilliant mind as it relates to his personal views on God and religion," Gazin said. "The personal nature of the letter and the timing of it in Albert Einstein's life adds to the implication of the certainty with which he wrote it."

Einstein experts say the letter supports the argument that the physicist held complex, agnostic views on religion. He rejected organized faith but often spoke of a spiritual force at work in the universe.

Einstein's most famous legacy is the special theory of relativity, which makes the point that a large amount of energy could be released from a tiny amount of matter, as expressed in the equation E=MC2 (energy equals mass times the speed of light squared).

The theory changed the face of physics, allowing scientists to make predictions about space and paving the way for nuclear power and the atomic bomb.