On the heels of the American Revolution came a Second Great Awakening. It did not transform the U.S. into a Christian nation either, but it did something perhaps more volatile.

In a republic with no established church, faith became a matter of choice, and with that came power. Ordinary Americans claimed the authority to interpret Scripture, experience God directly, and remake the world around them. Camp meetings dotted the landscape into the 19th century, preaching that salvation was open to all and that society itself could be redeemed. A sense of Christian duty birthed reform movements seeking to fix all manner of social ills.

But the same movement that inspired people to challenge injustice also gave others the tools to defend it. In the North, revivalism fueled crusades against slavery. In the South, it helped sanctify slavery, wr ...