I’m in the strange position of tracking less and less with Chesterton’s argument as he starts discussing Christianity more directly.
This is definitely an argument for Christendom primarily, and the doctrines of Christianity itself only indirectly, insofar as they produced Christendom. In chapter 1 Chesterton claims to be defending Christianity as defined in the Apostle’s Creed, but the focus is definitely on Christian societies and the benefits of Christianity to society.
In chapters 7 and 8, Chesterton makes the case that the ideals of Christianity are actually a better foundation for the goals of secularism, than secularism itself is. Christianity provides the best basis for the reforming of society, for liberty, for love, for democracy. The flow of the argument is wild and creative, as you can expect from Chesterton. But.
In the first place, presenting orthodoxy as nearly synonymous with Christendom, and with Christianity’s capacity for social change, seems to be a small version of Christianity. It is great that Original Sin grounds democracy and equality; and that the Trinity grounds our love for others. But are these the most important things that can be said about Original Sin and the Trinity? Strangely, in a book so focused on narrative and story, he is not telling the story of fall and redemption.
Second is the question of Christianity’s record at some of these social issues. Chesterton says “It does not matter how often humanity fails to imitate its ideal; for then all its old failures are fruitful.” But I think it matters a great deal, especially in an argument such as this, when Christianity fails to imitate its ideal. If Christendom bears little resemblance to the invincible force for social good that Chesterton describes (particularly from the vantage point of a secularist), the obvious conclusion is that Chesterton has mischaracterized Christianity, not that it has just failed to live up to its ideal.
I find the argument much more compelling as a call to Christians for social involvement, and a reminder of the resources we have to draw from to that end, than as a defense of Christianity against competing philosophies.