"None of these books have directed themselves to understanding the man as a thinker, and his connections as a thinker to the larger context of ideas in the 19th century," says Guelzo, author of "Abraham Lincoln: Redeemer President" (Eerdmans).
In part, that's because history has largely portrayed Lincoln as a rough-hewn Victorian-era version of gaffe-prone Minnesota Gov. Jesse Ventura.
"During his own day, many people viewed Lincoln as an uncouth barbarian in a coonskin cap who was ignorant, incompetent, or both," says Guelzo, a professor at Eastern College in St. David's, Pa. Today, many scholars continue to subscribe to "the myth that there was no intellectual life there to worry about."
But Guelzo, who has written for both Christianity Today and the Journal of the Abraham Lincoln Association, sees Lincoln as an important intellectual figure, and his exploration of the 18th president's inner world includes a thorough examination of Lincoln's religious faith--a subject that has been intensely debated ever since the days after his death, when the slain president was compared to both Washington and Jesus Christ.
Killed by John Wilkes Booth on Good Friday, Lincoln's role as a national deliverer was eulogized in hundreds of "Black Easter" sermons heard in churches all across the land. One preacher pointed out "the fitness of the slaying of the second Father of our Republic on the anniversary of the day on which (Jesus) was slain. Jesus Christ died for the world, Abraham Lincoln died for this country."
Many Americans believed Lincoln to be the "redeemer president" poet Walt Whitman had cried out for a decade earlier. So deep was clergymen's respect for Lincoln that they overlooked that fact that he had spent Good Friday in a theater, a place one Presbyterian preacher of the day described as "the very gates of Hell."
Understanding Lincoln's own views on religion is harder to deduce, and over the years he has been portrayed as a Bible-believing Christian, a Unitarian, and an atheist.
Guelzo portrays Lincoln as a complex and private man whose youth was largely shaped by Christian fundamentalism, but whose adulthood and political theories were more powerfully guided by Enlightenment ideals.
"Historians are dealing always in cases of probability, approximations, and assessments," says Guelzo, who says that questions concerning Lincoln's faith can't be answered with certainty.
"In part, that was because he was an exceptionally private man. Even his closest associates and friends remarked at how reticent and shut-mouthed this man was. In addition, he was aware of the possible consequences of talking about whatever religion he had, particularly if whatever religion he had was not orthodox."
According to Guelzo, Lincoln's inner world revolved around three main components: the strict Calvinism of his youth, the Enlightenment thinking he embraced in his early adult years and the classical liberal political theory that guided his presidency.
"Both of Lincoln's parents were members of an ultra-predestinarian Calvinist sect that believed so strongly in the sovereignty of God that they didn't believe in sending out missionaries," Guelzo says. "They believed that human beings don't make decisions about their future, but God makes those decisions and people submit to them."
Then in his early 20s, Lincoln devoured the works of Thomas Paine and some of the Enlightenment's most skeptical thinkers. After this point, Lincoln no longer considered himself to be formally religious, but he was still haunted by his upbringing.
Lincoln's political and economic thought stressed individualism, rationalism, capitalism and social mobility--all important components of Enlightenment liberalism.
"As president, Lincoln had great respect for the importance of cultural issues that were of importance to religious people, and he went out of his way to stress his commonality with evangelicals and support their issues," Guelzo says. "But like many present-day politicians, he felt the necessity of treading very carefully when questions of religion were raised."
Guelzo calls Lincoln "a typical Victorian doubter" like "Moby Dick" author Herman Melville. According to fellow writer Nathaniel Hawthorne, Melville could "neither believe or be comfortable in his unbelief."
Lincoln described himself as a doubting Thomas, an allusion to the disciple who could not believe in the resurrection of Christ "unless I see the nail marks in his hands."
Mary Todd Lincoln said that just before her husband was shot at the Ford Theater, he told her that he wanted to visit the Holy Land, a journey that many Victorians made to shore up their fractured faith.
"At the end of the Thomas story, Thomas does believe, but he required a remarkable demonstration," Guelzo says. "Lincoln never got that remarkable demonstration."