After Jesus Christ, however, there have been more books written about Abraham Lincoln than any other person who ever lived - 16,000 as of 2009:
[Source Craig Symonds, Author of "Lincoln, His Admirers"].
After Jesus, there have been more seekers, writers readers and admirers about Abraham Lincoln - the humble, countrified, never-attending-grade-school - much less college!
Abraham Lincoln, somewhat 'ill-bred' (crude, some declared!) but born and raised in a 'three-sided-log cabin' (the fourth open to the elements, protected by a fire for the panthers and bears prowling at night;
Yet Lincoln has been studied, written and red about more than any other human being:
>> more than any King, Pharaoh, Caesar, or Czar; Thus by so many searching-&-researching Lincoln’s Life, everything knowable should already be known:
>> more than any Dictator, Emperor, President or Prime Minister;
>> more than any Shah, Ayatollah, Imam or Generals of Armies;
>> more than any Scientist, Philosopher, Musician, Sports or TV Star;
>> In fact - according to some research - there have been more books about Lincoln than all other US Presidents: combined!
If you inquire about Abraham Lincoln's faith, you will find every position of faith possible thus attributed: Angry-Atheist, Super-Skeptic, Antagonistically-Agnostic, Doubting-Deist, Trusting-Theist, Effervescent-Evangelical, Fervently-Faithful . . . and ALL POSITIONS have a dozen personal "quotes" to prove their particular point of view.
LINCOLN CLAIMED CONSTANT PRAYER:
When a pious minister told Lincoln he "hoped the Lord is on our side," the president responded, "I am not at all concerned about that.... But it is my constant . . . prayer that I and this nation should be on the Lord's side."
[SOURCE: Carpenter, F.B. (1866). Six Months at the White House. p. 282].
[NOTE: This is an ONLINE BOOK For free - http://books.google.com/books?id=FTsl3N7hDpAC&printsec=frontcover&dq=six+months+at+the+white+house+carpenter&source=bl&ots=bUkvamYWdL&sig=rWzfEd_nifcgEQhd6T3EvRsKPqk&hl=en&ei=gXt_S4fMKIOeswPcq9n8Aw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBEQ6AEwAg#v=onepage&q=&f=false.
HIS WIFE CLAIMED HIM IN DAILY, HOURLY PRAYER:Mrs Lincoln. . .when Mr. Lincoln became elevated to Office - with the care of a great Nation, upon his shoulders - when devastating war was upon us then indeed to my knowledge - did his great heart go up daily, hourly, in prayer to God - for his sustaining power.
[SOURCE: Mary Todd Lincoln, letter to Rev. James Smith, June 8, 1870. As reported in Mary Todd Lincoln by Justin G. Turner and Linda Levitt Turner, pp 567-568].
According to cabinet member Salmon Chase, as Lincoln was preparing to issue the preliminary Emancipation Proclamation, Lincoln said, "I made a solemn vow before God, that if General Lee was driven back from Maryland I would crown the result by the declaration of freedom to the slaves." 
[SOURCE: Carpenter, Frank B (1866). Six Months at the White House. p. 90.].
Next came Robert E. Lee's impressive victory at the Second Battle of Bull Run, after which he said, "I have been driven many times upon my knees by the overwhelming conviction that I had nowhere else to go."
[SOURCE: Reed, James A. (July 1873). "The Later Life and Religious Sentiments of Abraham Lincoln". Scribner's Monthly 6 (3): 340].
http://books.google.com/books?id=KOYGAQAAIAAJ&pg=PA333&dq=%22Scribner%27s+Monthly%22+%2B1873+%2B%22The+Later+Life+and+Religious+Sentiments+of+Abraham+Lincoln%22&lr=&client=firefox-a#v=onepage&q=&f=false. citing Noah Brooks article in Harper's Monthly, July 1865
[SOURCE: David H. Donald, Lincoln (Simon & Schuster, 1995), 354] writes, "By the summer of 1862, Lincoln felt especially in need of divine help. Everything, it seemed, was going wrong, and his hope for bringing a speedy end to the war was dashed."
Following Lincoln's assassination, the well-known Presbyterian clergyman, the Rev. John H. Barrows, D.D., wrote and signed as follows: that Lincoln had become a Christian while in Washington.
"In the anxious uncertainties of the great war, he gradually rose to the heights where Jehovah became to him the sublimest of realities, the ruler of nations. (PROVIDENCE)
When he wrote his immortal Proclamation, he revoked upon it not only 'the considerate judgment of mankind,' but the 'gracious favor of Almighty God.'(PROVIDENCE)
When darkness gathered over the brave armies fighting for the nation's life, this strong man in the early morning knelt and wrestled in prayer (MAN of PRAYER!) . . .
. . . with Him who holds the fate of empires.(PROVIDENCE)
When the clouds lifted above the carnage of Gettysburg, he gave his heart to the Lord Jesus Christ.
When he pronounced his matchless oration on the chief battlefield of the war, he gave expression to the resolve that 'this nation, under God, should have a new birth of freedom.'
And when he wrote his last Inaugural Address, he gave it the lofty religious tone of an old Hebrew psalm.
[SOURCE: The Rev. John H. Barrows, D.D., well-known Presbyterian clergyman; "Lincoln Memorial Album". p. 508, Library of Congress, USA].
Lincoln's Second Inaugural Address in March 1865:
Both [North and South] read the same Bible, and pray to the same God; and each invokes His aid against the other.
It may seem strange that any men should dare to ask a just God's assistance in wringing their bread from the sweat of other men's faces; but let us judge not that we be not judged.
The prayers of both could not be answered; that of neither has been answered fully. The Almighty has His own purposes.
[SOURCE: Lincoln's Second Inaugural Address in March 1865:
His views seemed to settle so naturally around that statement, that I considered no other necessary. His language seemed not that of an inquirer, but of one who had a prior settled belief in the fundamental doctrines of the Christian religion.
Once or twice, speaking to me of the change which had come upon him, he said, while he could not fix any definite time, yet it was after he came here, and I am very positive that in his own mind he identified it with about the time of Willie's death.
DAILY PRAYER:He said, too, that after he went to the White House he kept up the habit of daily prayer. Sometimes he said it was only ten minutes, but those ten minutes with God he had. There is no possible reason to suppose that Mr. Lincoln would ever deceive me as to his religious sentiments.
In many conversations with him, I absorbed the firm conviction that Mr. Lincoln was at heart a Christian man, believed in the Savior, and was seriously considering the step which would formally connect him with the visible church on earth. Certainly, any suggestion as to Mr. Lincoln's skepticism or Infidelity, to me who knew him intimately from 1862 till the time of his death, is a monstrous fiction -- a shocking perversion.
[SOURCE: Reed, James A. (July 1873). The Later Life and Religious Sentiments of Abraham Lincoln. 6. Scribner's Monthly. p. 340].
Website: http://books.google.com/books?id=KOYGAQAAIAAJ&pg=PA333&dq=%22Scribner%27s+Monthly%22+%2B1873+%2B%22The+Later+Life+and+Religious+Sentiments+of+Abraham+Lincoln%22&lr=&client=firefox-a. Retrieved 2010-02-20. Noah Brooks to J.A. Reed, December 31, 1872