Here, we see some of the same ideas that appear in his (more interesting) work, Orthodoxy. Chesterton loves to revel in the extreme things of Christianity. How can it be against everything and for everything? It is furious in both direction. He loves this stuff and is always bringing it up.
Those who would suggest that the faith was a fanaticism are doomed to an eternal perplexity. In their account it is bound to appear as fanatical for nothing, and fanatical against everything. It is ascetical and at war with ascetics, Roman and in revolt against Rome, monotheistic and fighting furiously against monotheism; harsh in its condemnation of harshness; a riddle not to be explained even as unreason. And what sort of unreason is it that seems reasonable to millions of educated Europeans through all the revolutions of some sixteen hundred years? People are not amused with a puzzle or a paradox or a mere muddle in the mind for all that time. I know of no explanation except that such a thing is not unreason but reason; that if it is fanatical it is fanatical for reason and fanatical against all the unreasonable things.
That is the only explanation I can find of a thing from the first so detached and so confident, condemning things that looked so like itself, refusing help from powers that seemed so essential to its existence, sharing on its human side all the passions of the age, yet always at the supreme moment suddenly rising superior to them, never saying exactly what it was expected to say and never needing to unsay what it had said; I can find no explanation except that, like Pallas from the brain of Jove, it had indeed come forth out of the mind of God, mature and mighty and armed for judgement and for war.
G.K. Chesterton, The Everlasting Man, p.265
He even throws out some specific examples in this case:
How would Francis the Troubadour have fared among the Calvinists, or for that matter among the Utilitarians of the Manchester School? Yet men like Bossuet and Pascal could be as stern and logical as any Calvinist or Utilitarian. How would St. Joan of Arc, a woman waving on men to war with the sword, have fared among the Quakers or the Doukhabors or the Tolstoyan sect of pacifists? Yet any number of Catholic saints have spent their lives in preaching peace and preventing wars. It is the same with all the modern attempts at Syncretism. They are never able to make something larger than the Creed without leaving something out. I do not mean leaving out something divine but something human…
-p.202
At the end he reiterates that when someone introduces a heresy, it is more likely to make God less humanlike. It doesn’t add new ideas about God, but rather it takes away the human side of Christ. It separates the divide between the image of the creator and the creator himself. Beware of making God too ethereal. He has told us again and again we are more like him than we know.