Here’s help from H. L. Mencken:

Some time ago a publisher told me that there are four kinds of books that never, under any circumstances, lose money in the United States first, detective stories; secondly, novels in which the heroine is forcibly debauched by the hero; thirdly, volumes on spiritualism, occultism and other such claptrap, and fourthly, books on Lincoln. But despite all the vast mass of Lincolniana and the constant discussion of old Abe in other ways, even so elemental a problem as that of his religious faith surely an important matter in any competent biography is yet but half solved. Here, for example, is the Rev. William E. Barton, grappling with it for more than four hundred large pages in “The Soul of Abraham Lincoln.” It is a lengthy inquiry the rev. pastor, in truth, shows a good deal of the habitual garrulity of his order but it is never tedious. On the contrary, it is curious and amusing, and I have read it with steady interest, including even the appendices. Unluckily, the author, like his predecessors, fails to finish the business before him. Was Lincoln a Christian? Did he believe in the Divinity of Christ? I am left in doubt. He was very polite about it, and very cautious, as befitted a politician in need of Christian votes, but how much genuine conviction was in that politeness? And if his occasional references to Christ were thus open to question, what of his rather vague avowals of belief in a personal God and in the immortality of the soul? Herndon and some of his other close friends always maintained that he was an atheist, but Dr. Barton argues that this atheism was simply disbelief in the idiotic Methodist and Baptist dogmas of his time that nine Christian churches out of ten, if he were alive to-day, would admit him to their high privileges and prerogatives without anything worse than a few warning coughs.

As for me, I still wonder. (“Five Men at Random: Abraham Lincoln”)

The Congregationalist pastor, William E. Barton was, by the way, the father of Bruce Barton, the same writer who turned Jesus into a model business executive in The Man Nobody Knows