Here's a brief disucussion of the book from PBS --
Edward, I agree with your point about religion being a decisive element in the American Civil War. Strongly held religious or moral beliefs motivated three of the four conflicts which caused the war. William L. Ayers names these as 1) a struggle over a hypothetical railroad, 2) a novel written by an obscure woman, 3) an act of symbolic terrorism, 4) a media war over a distant territory (What Caused the Civil War: Relections on the South and Southern History, p. 139).
It's important to understand the moral beliefs of Northerners who advanced the belief of slavery as a great evil, indeed the greatest sin of all. After deciding to explore the "Battle Hymn of the Republic" some years ago, I read Deborah Pickman Clifford's biography of Julia Ward Howe, Mine Eyes Have Seen the Glory. Young Julia wrote of her early life, "I studied my way out of all the mental agonies which Calvinism can engender and became a Unitarian." Many of her anti-slavery friends and admirers in the North seem to have been Unitarian as well. However, best I can tell, John Brown was utterly Calvinist in his beliefs.
Using passages from the Bible concerning slavery, here's our problem. If scripture doesn't condemn in the institution and St. Paul and other New Testment authors accept it as part of a fallen world, we assume it's an acceptable social condition. The critical element for Bible-believing Christians is always the Word of God.
Other people who readily describe themselves as Christians don't use scripture that way. They treat the Bible like the lunchtime buffet after church -- people pick and choose what they please. Others, such as Henry Wordsworth Longfellow, Ralph Waldo Emerson and other Northern opinion-makers didn't regard the Bible as a significant moral authority, just a general guide. They regarded human reason and emotional urgings as preeminent for any moral judgement.
It would have been futile to argue Bible texts with people like them. That's probably still true today!