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."
http://www.usnews.com/articles/news/history/2009/02/12/abraham-lincolns-religious-uncertainty.html