There is actually doubt about whether Lincoln was a Christian. He frequently attended Presbyterian services with his family. He also was fond of the Bible, but there is debate as to whether he believed in the divinity of Christ. There is virtually no doubt that he believed in God though.

There were several 19th century presidents that were nondenominational. Obama is the first president without a particular religious affiliation since Rutherford B. Hayes. I wonder if Hayes had a Rev. Wright he had to run from?

The most common religious affiliation among US presidents belongs to the Episcopalians, the last of whom was George H.W. Bush.