Was Abraham Lincoln a Christian? Of course, in order to answer that question, you must go back and ask an even more fundamental question: What is a Christian? You can't determine if someone is a Christian, unless you have a working, baseline definition of the term "Christian."
There has been much debate over Abraham Lincoln's relationship with Christianity. What makes the debate even more controversial and never-ending is that it really depends upon which part of Lincoln's life one chooses to focus.
Early in life, there is little evidence that Abraham Lincoln embraced the fullness of the Christian faith. His detachment from any church and whimsical attitude toward matters of faith (which were borderline sacrilege - in the minds of many) stand as indications that Lincoln was not a committed Christian.
As Lincoln matured, however, things changed. With the death of two of his sons and the tragedy of the American Civil War, Abraham Lincoln became much more reflective, philosophical, and (yes) religious. And it's during this latter stage of his life, where many believe he made a decision to embrace Christianity.
In the end, only God and Abraham Lincoln know the full nature of Lincoln's faith, but this article by Mark Noll may shed some light on the question. Though Noll concludes (erroneously, I believe, since the evidence doesn't warrant such a definitive interpretation) that Lincoln was "not an orthodox, evangelical, 'born-again' Christian striving toward the 'higher life'", I nevertheless highly recommend you take time to read the article.