- A newly resurfaced letter raises questions about Abraham Lincoln's views on religion, faith and God.

- Lincoln was not religious for most of his life, but his faith seems to have evolved and progressed during his presidency.

- The letter went up for sale this week for $35,000.

Abraham Lincoln is known for many things. He led the nation through the Civil War, emancipated the slaves and delivered eloquent speeches about democracy and liberty. But Lincoln's religious views have long been a matter of debate.

Now, a newly resurfaced letter from the 19th century is raising questions once again about the 16th president's relationship with God. The three-page letter, which was written by Lincoln's old law partner, William Herndon, and just went up for sale for a price of $35,000, claims that Honest Abe was driven not by faith, but by politics.

Lincoln's attitude toward religion was clearly complex, and the discussion is far from over, as some evidence suggests that Lincoln's sense of faith evolved and deepened throughout the war and his presidency. Still, the letter offers a rare view of Lincoln's inner life from someone who knew him before he went to Washington.

PHOTOS: Faces of the Civil War

"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," Herndon wrote in the letter, signed Feb. 4, 1866, a year after Lincoln's assassination.

"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," continued the letter, addressed to Edward McPherson, Clerk of the U.S. House of Representatives. "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."

Born in a log cabin in Kentucky in 1809, Lincoln was raised as a Baptist but rejected organized religion and never joined a congregation as an adult. After trying out a variety of careers, including postmaster and surveyor, he taught himself to be a lawyer and moved to Illinois, where he eventually partnered with Herndon in 1844. The two remained close until 1861, when Lincoln left to begin his presidency.

Twenty years later, immediately after Lincoln's death in 1865, a flurry of biographies emerged, many of them attempting to Christianize Lincoln, said historian Ronald White, author of A. Lincoln: A Biography. Herndon wanted to set the record straight.

So, he embarked on a journey to collect oral histories about the real Lincoln, which would eventually lead to another biography, published years later with help from a collaborator. Herndon's letter to Congress was part of that effort.

The letter offers a rare view of Lincoln's inner life, from someone who knew him before he went to Washington.Library of Congress

"People debate Lincoln's religion today. It is often invoked in public debate and there is also active debate among scholars," said Nathan Raab, vice president of the Raab Collection, which acquired the letter from a very old private collection and put it up for sale this week. "This letter brings that debate into the foreground."

Herndon's personal experience with Lincoln was limited to his pre-presidential period, when religion was completely absent from his life. After Lincoln left for the White House, the two never met up again.

But the challenges of a presidency, the angst of the Civil War and the 1862 death of his 11-year-old son would push Lincoln to consider God in ways he never had before, said White, who added that religion is something most Lincoln biographers have skimmed over.

Lincoln's second inaugural address points to his eventual embrace of religion in midlife, White said. The speech, which was just 701 words long, mentions God 14 times and quotes the Bible four times, with two references to the Old Testament and two to the New Testament. In comparison, there were zero biblical references in his first inaugural and just one Bible quote in all previous inaugural addresses combined.

NEWS: How Civil War Photography Changed War

After his son's death, Lincoln also developed a strong relationship with a Presbyterian minister named Phineas Densmore Gurley. And after his own death in 1865, Lincoln's secretary John Hay found an untitled and undated document in Lincoln's desk that both questioned God's presence in the midst of the Civil War and offered affirmation that God was somehow a silent actor in the war. Hay called it: Meditation on the Divine Will.

"I'm arguing that Lincoln went on a remarkable faith journey that moves forward quickly and matures during his four years as president," White said. "He was not just dropping phrases from the Bible. In both the second inaugural and the Meditation on the Divine Will, he was dealing at a very deep level with profound religious questions."

The rediscovered letter is undoubtedly an important document about an important historical figure, experts said. It isn't necessarily earth shattering, but it adds depth to our knowledge of Lincoln's personal life.

"It confirms what Herndon wrote in his letters and his essay on Lincoln's religion, so we are not exactly discovering something new or scandalous," said Allen Guelzo, a historian at Gettysburg College in Penn., and author of Lincoln's Emancipation Proclamation: The End of Slavery in America. "But it is certainly a great letter, and puts into one long paragraph much of what Herndon would say about Lincoln's religion in the coming years."