(RNS) On the eve of the 150th anniversary of the start of the Civil War, a long-lost letter has surfaced that describes President Abraham Lincoln’s belief in God.

The Raab Collection of Philadelphia plans to sell a recently discovered letter written in 1866 by William Herndon, a Springfield, Ill., lawyer and Lincoln confidant.

“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,” wrote Herndon of the nation’s 16th president.

“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.”

The collection estimates the letter is worth $35,000.

Lincoln’s faith has long been an elusive topic for historians. He was never baptized, did not join a church and usually did not discuss his beliefs.

“In rare instances, he divulged his true feelings to one close friend, longtime confidant and law partner, William Herndon,” said Nathan Raab, vice president of the Raab Collection. “He did believe in God, however difficult it might be to easily define those beliefs.”