Reddit reviews From Bible Belt to Sunbelt: Plain-Folk Religion, Grassroots Politics, and the Rise of Evangelical Conservatism

You likely grew up in the weird fusion of American Southern culture and a specific brand (Baptist evangelical) of Christianity. Most Christians are not American. They're not even mostly white. They're not even mostly male.

Here's a good book about the history if anyone is really interested.

And here's another.

For quantitative analysis of American Christianity, see Stark and Finke, The Churching of America.

For narrative history of the rise of contemporary evangelicalism as a right-wing political entity, see Darren Dochuk, From Bible Belt to Sunbelt; and Daniel Williams, God's Own Party: The Making of the Christian Right.

On how Christians dealt with the issue of slavery, see Mark Noll, The Civil War as Theological Crisis.

But to give my opinion (studied this in grad school), no, that's horseshit. He's just making up something that allows him to reduce cognitive dissonance.