The Age of Reason; Being an Investigation of True and Fabulous Theology is a work written by English and American political activist Thomas Paine. It follows in the tradition of eighteenth-century British deism, and challenges institutionalized religion and the legitimacy of the Bible. Originally distributed as unbound pamphlets, it was published in three parts in 1794, 1795, and 1807. It was a best-seller in the United States, where it caused a short-lived deistic revival. British audiences, however, fearing increased political radicalism as a result of the French Revolution, received it with more hostility. The Age of Reason presents common deistic arguments; for example, it highlights what Paine saw as corruption of the Christian Church and criticizes its efforts to acquire political power. Paine advocates reason in the place of revelation, leading him to reject miracles and to view the Bible as "an ordinary piece of literature rather than as a divinely inspired text". It promotes natural religion and argues for the existence of a creator-God.
Most of Paine's arguments had long been available to the educated elite, but by presenting them in an engaging and irreverent style, he made deism appealing and accessible to a mass audience. The book was also inexpensive, putting it within the reach of a large number of buyers. Fearing the spread of what they viewed as potentially revolutionary ideas, the British government prosecuted printers and book-sellers who tried to publish and distribute it. Nevertheless, Paine's work inspired and guided many free thinkers.
Paine's book followed in the tradition of early eighteenth-century British deism. These deists, while maintaining individual positions, still shared several sets of assumptions and arguments that Paine articulated in The Age of Reason. The most important position that united the early deists was their call for "free rational inquiry" into all subjects, especially religion. Saying that early Christianity was founded on freedom of conscience, they demanded religious toleration and an end to religious persecution. They also demanded that debate rest on reason and rationality. Deists embraced a Newtonian worldview, and they believed all things in the universe, even God, must obey the laws of nature. Without a concept of natural law, the deists argued, explanations of the workings of nature would descend into irrationality. This belief in natural law drove their skepticism of miracles. Because miracles had to be observed to be validated, deists rejected the accounts laid out in the Bible of God's miracles and argued that such evidence was neither sufficient nor necessary to prove the existence of God. Along these lines, deistic writings insisted that God, as the first cause or prime mover, had created and designed the universe with natural laws as part of his plan. They held that God does not repeatedly alter his plan by suspending natural laws to (miraculously) intervene in human affairs. Deists also rejected the claim that there was only one revealed religious Truth or "one true faith"; religion could only be "simple, apparent, ordinary, and universal" if it was to be the logical product of a benevolent God. They therefore distinguished between "revealed religions" (which they rejected), such as Christianity, and "natural religion", a set of universal beliefs derived from the natural world that demonstrated God's existence (they were, thus, not atheists).