In the preface, Chesterton specifies his goal for the book: to explain how he personally established his faith in his personal theology.
He begins by explaining that human nature seems to betray itself. On the one hand, humans feel a strong desire to explore, to learn, and to grow through journeying into the unknown. On the other hand, humans feel a need for security and safety. He explains that sanity contains both sets of emotions. Chesterton explains that Christianity is a suitable belief system to mitigate the effects of that paradox.
Then, he turns his attention to religious authority and the issue of ethos. Instead of saying that the human mind is all or nothing, he says that truth goes far beyond human conception or perception. However, true reason is a dependable approach to understanding life, he feels. He defends reason, citing the arguments of important Catholic thinkers, including St Thomas and St Augustine. He explains that philosophical skepticism stems in his opinion from distrust in God's authorship of reality. He expresses fear that the future of human thought is to eliminate any belief in God from philosophy.
He ends the book by asserting beliefs that he strongly suggests as a kind of orthodoxy for a thinking man's Christianity: He says the universe is clearly constructed with artistic design, and since the human mind is an interpreting machine, clearly this reality is designed to be interpreted as art. He says beauty is a sign of divinity, and that in response to beauty, humans should attempt religion with humility and patience.
