'Orthodoxy' is, I think, one of the most important of Chesterton's books. The lasting importance of a book depends not so much on its literary qualities or on its popularity, but rather on the theme handled.
There are really two central themes handled in this book. One is of Fairyland, the other is of the defence of Christianity; not that it is either true or false, but that it is rational, or the most shuffle-headed nonsense ever set to delude the human race. The method of apology that Chesterton takes is one that would cause the average theological student to turn white with fear.
The theological colleges, excellent as they are in endeavouring to train efficient laymen into equally efficient priests, usually assume that the best way to know about Christianity is to study Christian books. It is the worst way, because these books are naturally biased in favour of it. It is better to study any religion by seeing what the attackers have to say against it. Then a personal judgment can be formed.
This is, I feel, the method that Chesterton adopts in his deep and original treatise, 'Orthodoxy,' which is more than an essay and less than a theological work.
The Chestertonian contention is that philosophers like Schopenhauer and Nietzsche have embarked on the suicide of thought, and that a later disciple to this self-destruction is Bernard Shaw.
In the same way these pseudo philosophers have attacked the Christian religion, 'tearing the soul of Christ into silly strips labelled altruism and egoism. They are alike puzzled by His insane magnificance and His insane meekness.'
As I have said, the method to realize the worth of Christianity is to read all the attacks on it. This is what Chesterton does. In doing so he discovers that these attacks are the one thing that demonstrate the strength of Christianity. Because the attackers reject it upon reasons that are contradictory to each other. Thus some complain that it is a gloomy religion; others go to the opposite extreme and accuse it of pointing to a state of perpetual chocolate cream; yet again it is attacked on grounds of effeminancy, it is upbraided as being fond of a sickly sentimentalism.
Thus it is attacked on opposite grounds at once. It is condemned for being pessimistic, it is blamed for being optimistic. From this position Chesterton deduces that it is the only rational religion, because it steers between the Scylla of pessimism and avoids the Charybdis of a facile optimism. Regarding presumably the early Church she has also kept from extremes. She has ignored the easy path of heresy, she has adhered to the adventurous road of orthodoxy. She has avoided the Arian materialism by dropping a Greek Iota; she has not succumbed to Eastern influences, which would have made her forget she was the Church on earth as well as in heaven. With tremendous commonsense she has remained rational and chosen the middle course, which was one of the cardinal virtues of the ancient Greek philosophers.
The Christian religion is, then, rational because attacked along irrational grounds; the Church is also reasonable because she has not been swayed by the attraction of heresy nor listened to the glib fallacies of those who always want to make her something more or something less.