A handwritten letter in which Albert Einstein grapples with the concept of religion has sold for AU$3.9 million.

The so-called “God Letter” was written in 1954 German by Einstein to philosopher Eric Gutkind in response to one of his works.

“This remarkably candid, private letter was written a year before Einstein's death and remains the most fully articulated expression of his religious and philosophical views,” a statement from auction house Christie's said.

The letter has become a key manuscript in the debate over the relationship of science and religion.

In the letter, Einstein takes issue with the belief in God.

“The word God is for me nothing but the expression and product of human weaknesses,” he writes, translated from the original German.

“The Bible a collection of venerable but still rather primitive legends.

“No interpretation, no matter how subtle, can [for me] change anything about this.”

The world-changing physicist also muses on his own Jewish identity, seeing his own “like all other religions, an incarnation of primitive superstition”.

“The Jewish people to whom I gladly belong, and in whose mentality I feel profoundly anchored, still for me does not have any different kind of dignity from all other peoples,” he writes.