William E. Barton 
Soul of Abraham Lincoln [PDF ebook] 

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. So much has been said, and much of it with such intensity of feeling, on the subject of Lincoln’s religion, that a number of the more important biographies, including the great work of Nicolay and Hay, say as little on the subject as possible.