After finishing this book I agree with Mansfield that, based on the recollections of persons close to Lincoln presented in this book and Lincoln’s usage of the word ‘ God ‘ in his Emancipation Proclamation and in his 2nd Inaugural Address etc Abraham Lincoln grew to believe in the existence of a Christian God.
What is striking is his deliberate omission to use the words Jesus Christ or the Holy Ghost or the Trinity. This has not been addressed by Mansfield in his otherwise excellent book.