After Abraham Lincoln was fatally shot on Good Friday in 1865, American preachers forgot their prepared Easter sermons and rhapsodized about the martyred president. Some compared him to Moses or even Christ. Ever since, religionists have portrayed Lincoln as an exemplar of Christian faith.

But he wasn't, not in any conventional sense.

So reports Allen C. Guelzo, professor of American history at Eastern College in St. Davids, Pa., in "Abraham Lincoln: Redeemer President," published this week by Eerdmans.

Guelzo, a 46-year-old Episcopalian, has been intrigued by the 16th president since he narrated Aaron Copland's "Lincoln Portrait" for his Pennsylvania high school orchestra. And yet, the professor said in an interview, "I thought of him as a secular figure with relatively few intellectual interests." After all, Lincoln received only a few winters of formal schooling.

But in 1994, Guelzo researched American thinking about free will and discovered that Lincoln gave considerable attention to that and other theological topics. That roused Guelzo to write what's billed as the first intellectual biography of the man about whom more has been written than any other American.

The religious aspect of the tale, in a nutshell: Lincoln was unable to believe, but he was never comfortable in his unbelief.

Youthful skepticism gave way to deeper respect for religion. And during the devastation of the Civil War, Lincoln's self-made theology reshaped American history.

The key to Lincoln's belief system was a rough-hewn version of predestination that he absorbed from his parents' churches.

This doctrine, most common in the Calvinist or Reformed Christian traditions, teaches that God chooses in advance those who will be saved.

In Kentucky, Lincoln's parents were devoted to the hard-shell Primitive Baptists; later in his boyhood, in Indiana, his father and stepmother joined the slightly more moderate Separate Baptists. Both were rigidly Calvinistic.

Young Abe had little interest in his parents' churches. And while living in New salem, ill., in 1834, he wrote a "Little book on Infidelity" that contemporaries said attacked the divinity of Jesus. Then 25, he considered publishing it, but friends persuaded him to burn it.

That was fortunate, because Lincoln was launching a political career. As a champion of small and big business, his natural home was the Whig Party. Like today's Republicans, the Whigs drew heavy support from Evangelical Protestants--roughly equivalent to today's religious right--who wanted public piety and the abolition of slavery.

Lincoln would have gained politically by joining some church, maintaining a religious front and keeping his doubts to himself. But that would have been completely out of character. "He was, quite literally, honest Abe," Guelzo said.

Religion became a hot issue in 1846 when Lincoln won a seat in the U.S. House over Democrat Peter Cartwright.

"That I am not a member of any Christian Church, is true," Lincoln wrote in a handbill days before the election. But he denied any disrespect of religion in general or any Christian group.

When young, he said, he became inclined toward the "Doctrine of Necessity--that is, that the human mind is impelled to action, or held in rest, by some power over which the mind itself has no control."

Four years later, his wife, Mary, devastated by the death of their son, Edward, joined a Presbyterian church and became an active member. Lincoln had many chats with Springfield pastor James Smith, who recounted that unlike most skeptics, Lincoln was "a constant reader of the Bible."

Smith said Lincoln believed that some form of Providence was at work in the universe, but was unable to believe in a personal God or in Jesus as his savior.

Guelzo believes Lincoln combined two classic strains in American culture. He personifies the hope that in a market-oriented democracy, the poorest citizen can prosper through ambition.

But in addition, Americans "want our mobility linked to a lofty set of principles, not just profit," Guelzo said, and Lincoln also represents firm moral commitments.

Early in the Civil War, Lincoln came to believe that defeat for the North was inevitable if the war was waged only to save the Union, not for a higher moral cause.

At a crucial Cabinet meeting after the Battle of Antietam in September 1862, Lincoln astounded his colleagues by saying he had made a vow to himself and--he added after a pause--"to my Maker": If God allowed the North to repel Confederate Gen. Robert E. Lee's invasion, it would be Lincoln's duty to abolish slavery.

This prewar skeptic was now, Guelzo writes, "offering as his reason for the most radical gesture in American history a private vow fulfilled in blood and smoke by the hand of God."