Throughout the book he relates the Enlightenment to Protestant Christianity, for it is out of the clashes and reconciliations between those two systems that 19th-century American culture--a culture that lasted almost to our own time--took shape. Defined so broadly, the religion of Enlightenment obviously included many different kinds of people--deists and skeptics and liberal Christians, aristocrats and democrats, conservatives and revolutionaries. May divides the European Enlightenment into four major categories, and shows how each had a different effect in America. Obviously some ideas could be transmitted more easily than others to a society overwhelmingly Protestant and rapidly becoming democratic. May shows how the Enlightenment affected the thoughts and actions of major figures like Jefferson, Franklin, and John Adams, but these familiar figures are treated against a background of less well-known people--doctors and ministers, scientists and planters and politicians.