It is a plea for Christians to become advocates for human rights and justice. These, they argue, are central to Kingdom mission, and not secondary activities.
Interestingly, although neither author is Wesleyan, they refer to the Wesleyan Methodists of 1843, who gave themselves to the abolition of slavery and women's rights.
On the Side of the Angels stretched me out of my comfort zone. I disagree with a few of their points, but the general theme is an important correction to the evangelical church.
Reading this book, I recalled these words of John Wesley:
"The Gospel of Christ knows of no religion, but social; no holiness but social holiness. 'Faith working by love' is the length and breadth and depth and height of Christian perfection."
Along the same line, here's an interesting article by Keith Drury: The Holiness Movement's Heritage of Social Action