The 27 letters sold at auction at the California-based auction house Profiles in History this week are the largest cache of Albert Einstein’s correspondence ever offered for sale. Accumulated by a private collector over decades, they offer a rare glimpse into the scientist’s parenting style, personal relationships and spiritual beliefs--among other things.

Written longhand and typewritten, in both English and German, the 27 letters sold at auction this week range from the parental (Einstein urging his son to study harder in geometry) to the poignant. Writing to wish his uncle a happy 70th birthday, Einstein recalled how a toy steam engine the uncle had given him helped spark his passion for science. In one letter, he counsels a female friend who had recently discovered her husband had been unfaithful to her.

On a more serious note, Einstein twice corresponded with a man who wrote to him in the 1940s on the subject of God. Einstein, who was widely believed to be an atheist, clarified his views in the letters. “I have repeatedly said that in my opinion the idea of a personal God is a childlike one,” he wrote, “You may call me an agnostic, but I do not share the crusading spirit of the professional atheist….I prefer an attitude of humility corresponding to the weakness of our intellectual understanding of nature and of our own being.”

Einstein’s religious beliefs have been hotly debated over the years; he was Jewish, but wrote that he had gradually lost his faith in childhood. Many believed him to be an atheist, based in part on another letter he wrote a year before his death, in 1954, in which he called religion “an incarnation of the most childish superstitions.” That letter fetched a record 170,000 pounds ($265,000) when it went on auction in London in 2008. The evolutionary biologist and outspoken atheist Richard Dawkins, who had claimed Einstein as a fellow non-believer, was one of many unsuccessful bidders.

Buyers at this week’s California auction, whose identities are kept confidential by the auction house, paid a range of $6,875 to $62,500 for the individual letters in the collection. A letter in which Einstein wrote to his son discussing the connection between his theory of relativity and the atomic bomb fetched the highest price.

Joseph Maddalena, the founder of Profiles in History, called the Einstein letters “among the most important things I’ve ever handled.” As he told the Associated Press of Einstein: “We all know about what he accomplished, how he changed the world with the theory of relativity. But these letters show the other side of the story. How he advised his children, how he believed in God.”

Einstein’s letters were not the only historically significant items up for sale in the auction this week. Also on the block was a handwritten copy of the 13th Amendment to the Constitution, which abolished slavery in the United States. Signed by 125 members of Congress in 1865, the document sold for a cool $187,500.