By Emanuella Grinberg, CNN

(CNN) - President Abraham Lincoln was a "theist and a rationalist" who doubted "the immortality of the soul," a close friend said in a letter that provides a rare, intimate glimpse into the Civil War president's religious views.

"Mr. Lincoln’s religion is too well known to me to allow of even a shadow of a doubt; he is or was a Theist - a Rationalist, denying all extraordinary -– supernatural inspiration or revelation," William H. Herndon wrote in a letter dated February 11, 1866, to Edward McPherson, clerk of the U.S. House of Representatives.

Herndon was one of Lincoln's closest friends. The two met in Springfield, Illinois, and practiced law together for 17 years before Lincoln became president in 1861. After Lincoln's assassination on April 14, 1865, he authored "Herndon's Lincoln," a biography based on contributions from Lincoln's friends and contemporaries considered among the most authoritative for its proximity to the elusive president.

The three-page letter, which is being offered for sale by the Raab Collection for $35,000, offers a rare account from someone close to Lincoln on the subject of his religious beliefs - a topic that has eluded historians. Lincoln did not discuss his religious beliefs and he did not belong to a church.

"Lincoln was reticent to discuss religion, particularly after his election, which has fueled the ongoing debate about whether he believed in God or if he was Christian in the way we would explain it today," said Nathan Raab of the Raab Collection. "These are subjects still being debated."

His early religious outlook was colored by the evangelical Baptist faith of his parents and a Calvinist theology of predestination - the belief that the fate of all men and women had been predetermined by God, PBS.org said of Lincoln in its "God in America" series. Lincoln rejected this Calvinist view later in life and shunned emotional excess, but the Calvinism of his youth left him with a sense of fatalism that endured throughout his life.

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Lincoln's views on providence and God's will in the context of the Civil War have been the source of great scrutiny over the years.

In his letter, written less than a year after Lincoln's death, Herndon wrote that the president was "the purest politician."

"At one time in his life, to say the least, he was an elevated Pantheist, doubting the immortality of the soul as the Christian world understands that term. He believed that the soul lost its identity and was immortal as a force. Subsequent to this he rose to the belief of a God, and this is all the change he ever underwent. I speak knowing what I say. He was a noble man - a good great man for all this," he wrote.

"I love Mr. Lincoln dearly, almost worship him, but that can’t blind me. He’s the purest politician I ever saw, and the justest man. I am scribbling - that’s the word - away on a life of Mr. Lincoln - gathering known-authentic - true facts of him."