From a piece by Timothy George, republished in First Things from The Birmingham News: 

Though Abraham Lincoln was neither baptized nor joined a church of any kind, he was the most spiritually minded president in American history… So why did he never join a church himself? Two reasons. First, he was offended by the religious rivalry and braggadocio of the frontier preachers of his day. None of them made a compelling case to his lawyerly mind that only one denomination was right and all the others wrong. Further, Lincoln was reticent, “the most shut-mouthed man I know,” as his law partner William Herndon said. He did not want to cross the thin line between sincerity and self-righteousness. There was nothing smug about Lincoln’s faith.

Read the rest here.