Where there may be truth in that, the South, in some sense, fought for slavery 'cause the North was prosperous as an industrializion and, in many parts, the South depended on the North because of that. Much of the South was rural and dealt mainly in farming, and the slaves were a "commodity" that they felt they couldn't let go of and not be beholding to the North. History does show after the war ended that the South, without slavery, became overly dependant on the North. And much of the South was not for Reconstruction.
This is not to say that religion and slavery as a whole didn't have their roles; also expansion of the West played a part, if the Confederacy won the war, it would've been more likely they may have had control of that region, and the North couldn't allow that. The only people, I feel, who were fighting for religion/slavery were those who owned slaves; everyone else, most likely, were fighting for their homes, and their way of life which they believed (mainly through propaganda) the North wanted to take away from them.
In Southern states today, I feel that still holds true for many.