The major idea of the Enlightenment was that all things ought to be looked at through the lens of reason. People should examine ideas logically, not simply accept or reject them based on what religion or tradition told them. Both Rousseau and Voltaire espoused ideas that were very much in this tradition.
Voltaire is best known for his belief in religious toleration. This is very much an Enlightenment idea. Reason cannot reveal to us which religion, if any, is correct. Therefore, it does not make sense to force people to believe in any given religion or to condemn them for believing in some religion that is seen as "wrong."
Rousseau is best known for his idea of the social contract. This emphasized the idea that government was only legitimate if the people consented to be governed. To Rousseau, there was no logical reason to go on following kings, for example, simply because tradition held that they had a divine right to rule. This, too, is in keeping with Enlightenment ideas.