Short answer: probably (at least up to 1862).
Longer answer: Abraham Lincoln's religious views are hard to ascertain as he was very cagey about them, probably knowing that they could be used to damage him politically. However, particularly earlier in his life, they seem to be a mixture of scepticism about organized religion accompanied by non-religious fatalism.
In 1846, when Lincoln ran for Congress against an evangelist, who tried to smear him as an 'infidel,' Lincoln defended himself, without denying the specific charge:
"That I am not a member of any Christian church is true; but I have never denied the truth of the Scriptures; and I have never spoken with intentional disrespect of religion in general, or of any denomination of Christians in particular ... I do not think I could myself be brought to support a man for office whom I knew to be an open enemy of, or scoffer at, religion."
Lincoln's secretary, his bodyguard, and some of his close friends seemed to think that Lincoln was an 'infidel.' His wife, however, denied that he was. Much of the other evidence that Lincoln became a Christian in later life seems to be from sources with a vested interest, such as clergymen.
When Lincoln's son died, he was deeply grieved, and during that time made some religious statements and also tried spiritualism. It is possible that he became more religious after this time either because of his son's death or the Civil War.
More questions on atheism:
- Do credible historians agree that the man named Jesus, who the Christian Bible speaks of, walked the earth and was put to death on a cross by Pilate, Roman governor of Judea?
- What are some alternatives to the heaven/hell/nothing paradigms for what happens to our consciousness after we die?
- Was Steve Jobs religious at all?