Orthodoxy is Chesterton's answer to a challenge. After his book Heretics attacked the philosophies of his contemporaries, critics replied that he had never stated his own. He responds with what he calls a sort of slovenly autobiography: the story of how he tried to build a philosophy out of his own speculations and discovered, like a yachtsman who sails off course and lands on England believing it a new island in the South Seas, that his invention already existed. He set out to found a heresy of his own, and when he put the last touches to it he found it was orthodoxy, by which he means the Apostles' Creed.
He begins at the madhouse. The danger to the mind is not imagination but reason working without root: the madman has lost everything except his reason, and his theory, like the materialist's, explains everything while remaining somehow too small, a perfect but narrow circle, a clean and well-lit prison of one idea. Modern thought then commits suicide. Humility has moved from ambition to conviction, so men doubt the truth instead of themselves, and skepticism carried through ends by doubting the validity of thought itself. Reason, Chesterton insists, is itself an act of faith, and mysticism, which lets one thing stay dark so that everything else can be light, is what has always kept men sane.
The positive doctrine comes from the nursery. Fairy tales hold logical necessities to be binding even in elfland, but they treat nature's regularities as magic: one can imagine trees growing golden candlesticks, so fruit is a wonder rather than a law. Repetition in nature may be the encore of a God strong enough to exult in monotony, saying 'Do it again' to the sun every morning. In fairyland happiness always hangs on a condition one did not set, which Chesterton names the Doctrine of Conditional Joy, and existence itself is so eccentric a gift that its conditions cannot be called unjust. He adds a cosmic patriotism: we owe the world a loyalty prior to admiration, loving it enough to change it, and the doctrine of the Fall then explains how the world can be precious and yet wrecked. When that spike of dogma fitted the hole he had found in the world, the rest of the creed clicked into place bolt after bolt.
The central chapter argues that the world is nearly reasonable, but not quite, and that Christianity is shaped to that near-miss. Critics attack the faith for opposite vices at once, as too gloomy and too hopeful, too meek and too warlike, which suggests the critics rather than the creed may be deformed. Where pagan ethics balanced by mixture, Christianity keeps furious opposites side by side at full strength: the criminal must be forgiven seventy times seven while the crime is not forgiven at all, and the lion lies down with the lamb without losing its royal ferocity. Holding that balance through history, swerving past heresy after heresy, was in his image one whirling adventure, with the heavenly chariot flying through the ages and the wild truth reeling but erect.
The closing chapters defend fixity and authority. Progress is meaningless without a fixed ideal: we are not altering the real to suit the ideal but altering the ideal, which is easier, and since things left alone decay like the white post that must be repainted, even keeping anything demands a standing revolution. Theologies that dissolve doctrine sap reform, he argues; it is the transcendent God, original sin, and souls in real peril that make divine discontent and action possible. He accepts the authority of the creed because evidence converges on it from many directions and because it keeps answering instincts it did not create. The book ends with joy, the gigantic secret of the Christian, and with the one thing Chesterton fancies Christ hid from men: his mirth.