Leviathan: Or the Matter, Form, and Power of a Commonwealth, Ecclesiastical and Civil

It's clear that the project of the Enlightenment was the dialectic of answering the pessimism of Hobbes with the optimism of John Locke. They might not have had to agree with Hobbes, but they had to respond to him.
Hobbes is very subtle in some of his arguments. The time period still believes in witches and superstition, and beliefs based on authority. Hobbes does not. The strength of Hobbes arguments is he understands what knowledge is, what absolute knowledge means, and how misleading 'belief in' and 'faith' are as foundations for understanding and explaining. Hobbes has an advanced way of putting the nature of knowledge on a firm foundation. Chronologically Hobbes comes after Bacon and Galileo but before Newton, and is laying a foundation for Newton to think of knowledge as Universal, Necessary and Certain for explaining and understanding the natural world. (Newton with his laws of gravity will see the world in those terms as will Kant, but Heidegger and Kuhn much later will show how knowledge is best thought of as particular, contingent and probable).
Hobbes is also not very subtle in some of his arguments. He is an absolute authoritarian when it comes to the power vested in the sovereign. I realize now why Richard Nixon in the David Frost interviews argued that the chief executive could not possibly break the law and why some would think the Nuremberg defense was reasonable (Hobbes would defend both). Even in all his muddle arguments sometimes real gems of wisdom pop out. For example, no man can be trusted when his job is on the line, or those who are influential and widely known are more responsible for inciting violence than those who are unknown.
There are two things I know with certainty: 1) everyone should listen to (or read) this book, 2) nobody should listen to the third chapter of this book (probably 8 hours long), the one on the commonwealth of the church. The third chapter had way too many bible verses and I would say that the Enlightenment ignored that chapter for the most part with one exception. I had just recently read the "Age of Reason" by Thomas Paine, and it is obvious to me that in his first edition of the book he took Hobbes chapter three and used that to show why the bible was not holy and he also directly restates Hobbes points about how revelation from God most always be second hand to everyone but the person who speaks directly to God. An aside, it seemed obvious to me that Hobbes was an atheist (or deist), but he never would be explicit about it. Though, in that dreaded third chapter (please skip it if you can!) he is really interested in showing why the "papist' have less authority than the Church of England.
I really can't understate the obvious importance that this book has in the jump starting of the Age of Enlightenment. The book itself can be dry (and it is dryly read), but sprinkled through out the book are real pearls of wisdom. This book deserves to be the seventh most cited book on college syllabi. My only hesitation in recommending this book is the 3rd chapter with its endless bible citations. Don't skip the fourth chapter. There is a real elegant refutation against Aristotelian thought and the danger of appealing to authority over reason. After all, the Age of Enlightenment is most readily described by rejecting authority as your primary source for knowledge and appealing to reason instead.