Orthodoxy is a book by G. K. Chesterton.

This is one of the best apology of the Christian religion in contemporary history. It was written after the publishing of another Chesterton’s book, Heretics when the author had been accused of encouraging the readers to appeal to his philosophy whereas such philosophy didn’t exist. Orthodoxy became the book of such philosophy. Chesterton wrote, “...It is the purpose of the writer to attempt an explanation, not of whether the Christian Faith can be believed, but of how he personally has come to believe it. The book is therefore arranged upon the positive principle of a riddle and its answer. It deals first with all the writer's own solitary and sincere speculations and then with all the startling style in which they were all suddenly satisfied by the Christian Theology...”