That was particularly true about religion. “I don't Know anything about Lincoln's Religion,” grumbled David Davis, Lincoln’s personal attorney and his first appointee to the U.S. Supreme Court, and “don’t think anybody knew.”

Herndon, however, had been around Lincoln long enough to hazard a few guesses. First, the Lincoln he had known did “not believe…as a rational man…that the Bible was the peculiar, only, and special revelation of God.” As an ambitious young lawyer, he acquired a reputation for mocking religion, and even wrote “a pamphlet attacking the divinity of Christ special inspiration revelation &c.” Lincoln never joined a church, was never baptized, and never made any profession of belief.

And yet there were secrets about Lincoln’s religion about which even Herndon’s guesses failed. Lincoln had been raised in a family of pious, ultra-Calvinist Baptists, soaking up a substantial amount of sermons, hymns, and Scripture. He might not have believed “as a rational man…that the Bible was the peculiar, only, and special revelation of God,” but according to Herndon, Lincoln remained “superstitious, believed more or less in dreams, consulted negro oracles, had apparitions and tried to solve them.” And as he grew older -- and wiser about things that antagonize voters -- he agreed with the “absolute necessity of some form of Christianity, and never did, after reflection, attempt to disturb any man’s opinion,” his law partner wrote.

Lincoln’s election to the presidency, just in time to see the country fall into civil war, presented him with a different set of challenges to his meager stock of religious belief. Lincoln expected a quick and direct restoration of the Union. But in battle after battle, the Union armies were handed humiliating defeats. The president could make no logical sense of this apparent contradiction of progress. After a year-and-a-half of seemingly fruitless bloodshed, he concluded that God had taken a direct hand in events to stymie the war’s progress so long as it was waged for purely political purposes, and to force Lincoln to recognize that the war must be turned in a moral direction that spoke directly to the crime of slavery.

This insight is what eventually drove Lincoln to depart from the policy direction with which he had begun the war, and to issue the Emancipation Proclamation. To the astonishment of his Cabinet, Lincoln explained that his decision to issue the Proclamation was a “vow” he had made “to myself, and...to my Maker.”

In his Second Inaugural Address, Lincoln came as close to preaching a sermon as any U.S. President has ever dared utter. The war, he said, was not a struggle between a righteous Union and evil slaveholders – both North and South had been complicit in the crime of slavery, and the war was a judgment that God had chosen to bring on the entire nation “until all the wealth piled by the bond-man’s two hundred and fifty years of unrequited toil shall be sunk, and every drop of blood drawn with the lash, shall be paid by another drawn with the sword.” Only by submitting to that national judgment, Lincoln told his audience, could “malice toward none” and “charity for all” follow.

Lincoln’s public role as president embodied a profound struggle to preserve a “government of the people” that nevertheless still respected a core of moral truth which not even “the people” could repeal. And in that role, he reminds us that religion is neither alien to public life (to be locked up in private) nor a jack-in-the-box (ready to jump into every situation). It is, instead, a reminder of the feebleness of our own wisdom, and of the costliness of truth.

Allen C. Guelzo is Director of Civil War Era Studies and the Henry R. Luce Professor of the Civil War Era at Pennsylvania’s Gettysburg College