Unlike that of recent American presidents, so eager to testify about their "come to Jesus" experiences, the exact nature of Abraham Lincoln's religious faith is hard to pin down.

In early campaigns for Congress, opponents were able to tar him as a "scoffer" of religion. But Lincoln emphatically denied the charge, saying he couldn't vote for an enemy of religion. Lincoln is the only U.S. president who never joined a church, but he read the Bible frequently, and he told a friend in the year before his death: "Take all of this book upon reason that you can, and the balance on faith, and you will live and die a happier and better man."

And though Lincoln rejected the God's-on-our-side certainty of Northern abolitionist preachers, he eventually came to see the Civil War as divine retribution for the national sin of slavery. "Lincoln is clearly a believer in God and providence, yet it's a more mysterious God than his contemporaries worshiped," says Mark Noll, a history professor at the University of Notre Dame.

Beyond his reticence about personal matters and his obvious discomfort with organized religion, Lincoln's faith life eludes easy description because it changed dramatically during the White House years. If Lincoln arrived in Washington as an Enlightenment deist who, like the Founding Fathers, perceived a distant creator who left his creation to its own devices, the crucible of the Civil War made him believe in a justice-seeking God who intervened in history, even if his intentions were difficult to read.

"Because he never became a convert or joined a church, some people say he never changed," says Allen Guelzo, author of Abraham Lincoln: Redeemer President. "But you can't read about how Lincoln talks about God in speeches and documents after 1862 and say he's an infidel. Something had changed."

For Lincoln, growing up on the northern edge of the Bible Belt, the Good Book was one of the few volumes in the family collection. Lincoln read it closely enough to sprinkle conversations with biblical allusions. But he rejected his parent's Baptist faith as too emotional and chafed at the noisy denominational battles that had Methodists, Baptists, Universalists, and others denouncing one another on Sundays. Lincoln preferred the Enlightenment-fueled rationality of writers like Thomas Paine. Says Guelzo: "As a young man, Lincoln has a reputation as a village atheist."

His failure to clinch the Whig nomination for Congress in 1843 taught Lincoln that that reputation had political consequences, according to biographer Richard Carwardine. "[I]t was everywhere contended that no Christian ought to go for me, because I belonged to no church," Lincoln remarked after his loss. His secular reputation "levied a tax of considerable per cent upon my strength throughout the religious community."

Faith-based attacks against Lincoln returned in 1846, when he faced Methodist preacher Peter Cartwright in a race for Congress in Illinois. Lincoln denied claims that he looked down on religion but pointedly refused to defend his personal piety: "I have never denied the truth of the Scriptures; and I have never spoken with intentional disrespect of religion." This time, he prevailed.

Fifteen years later, in his first presidential inaugural address, Lincoln struck an overtly religious tone. "Intelligence, patriotism, Christianity, and a firm reliance on Him, who has never yet forsaken this favored land," Lincoln said in acknowledging the cracks that had begun emerging between North and South, "are still competent to adjust, in the best way, all our present difficulty."

What had changed? Lincoln had started attending Presbyterian services in 1850, following the death of his 4-year-old son, Edward. And once elected to the White House, Lincoln kept up his churchgoing ways, finding solace in what scholars call Old School Presbyterianism: a conservative, God's-in-charge brand of Christianity that rejected the political activism—including the abolitionist stance—of the new revivalism.

The onset of the Civil War and its spiraling death toll, along with the death of a second son, Willie, in 1862, deepened Lincoln's faith, even as that faith continued to defy categorization. (Lincoln declined to formally join the Presbyterian Church, even after his wife did.) "The failure of the North to thwart the South had shaken his self-confidence," says Guelzo. "The only confidence he could find was in a providential, interventionist God who had something up his sleeve, and that something was Emancipation."

Indeed, when he announced his decision to issue the Emancipation Proclamation to his cabinet on the heels of the much-needed Union victory at Antietam in September 1862, Lincoln said he was honoring a promise he made to God in exchange for a battlefield win. "God had decided this question in favor of the slaves," Lincoln told them, according to one account. A stunned cabinet member asked the ever rational Lincoln to repeat himself.

As the casualties on both sides mounted, Lincoln privately hashed his Civil War theology out on paper. In an undated writing thought to be from 1862 and never meant for publication, Lincoln describes a God whom neither abolitionists nor Confederates could claim as their own. "I am almost ready to say this is probably true—that God wills this contest, and wills that it shall not end yet," Lincoln wrote, in a piece his secretary later titled "Meditation on the Divine Will." " . . . He could give the final victory to either side any day. Yet the contest proceeds."

Though he attended a Christian church, Lincoln's God hewed closer to the Old Testament's ruler of nations. "His concept was that God calls nations to repentance just as he calls man and woman to repentance," says Joe Wheeler, author of Abraham Lincoln, a Man of Faith and Courage.

Some historians believe the undated "Meditation on the Divine Will" provided the basis for Lincoln's second inaugural address, the most overtly religious inauguration speech in American history. Fewer than 800 words long, the speech managed 14 mentions of God, four biblical allusions, and three invocations of prayer. "I read the previous 18 inaugurations and was surprised that they all mentioned God in the last paragraph, as a kind of add-on," says Ron White, author of the new A. Lincoln: A Biography . "Lincoln's mentions are not ornamental. They're part and parcel of the very strength of his argument."

That argument was that the Civil War was America's divine punishment. "If we shall suppose that American Slavery is one of those offences which . . . He now wills to remove," Lincoln told a war-weary nation, "and that He gives to both North and South this terrible war as the woe due to those by whom the offence came, shall we discern therein any departure from those divine attributes which the believers in a Living God always ascribe to Him?"