What institutions did Enlightenment thinkers criticize, and what was the nature of that criticism?
Enlightenment thinkers, who believed that reason would lead to universal and objective truths, criticized the institutions of absolute monarchy and the established church [the Catholic Church], which were the controlling sources of government and learning. This criticism was based upon the abuses of both institutions. The resulting great collective project of the age was L'Encyclopédie, of which Voltaire, Diderot, Montesquieu, and Rousseau were contributors. This work was a declaration of intellectual independence for men of all social classes in Europe which would allow the inquisitive to learn without having to rely upon the nobleman or the priest. The thinking of the Enlightenment philosophes helped to effect the French Revolution.
The Enlightenment was the Age of Reason; the leading thinkers were French: Voltaire, Diderot, Jean-Jacques Rousseau. Rousseau's social contract, the proposal that legitimate political authority must emanate from a social contract agreed upon by every citizen with the purpose of the mutual protection of all.
Thus, the group collectively is more important than each individual that makes it up. The sovereign and the general will are more important than its subjects and their particular wills.
England, too, had its share of great thinkers during this period as well: Thomas Hobbes, John Locke, David Hume, and Jeremy Bentham. Of these men, John Locke is the most prominent. Known as "The Father of Classical Liberalism." Like Rousseau, he believed that limits must be placed upon government. His theory of the natural rights of men placed emphasis upon the individual and argued that governments should have only limited powers. Like Rousseau, Locke believed that once a government usurps too much power, it should be overthrown by its citizens. Locke was also a proponent of religious tolerance.
Very important, too, is the German philosopher Immanuel Kant, who made monumental contributions in ethics, aesthetics, epistemology, and metaphysics; for, Kant's writings influenced nearly every philosophical movement that followed his time, making him one of the central personages of modern philosophy.