Why would Isaac Newton (as an enlightenment thinker) be considered significant in the history of Western civilization?
Isaac Newton's theories fundamentally changed how man views (or may view) the world. No longer was mysticism or faith an acceptable way of dealing with questions that seemed beyond man's reach: even God (if he existed) could be understood by applying rationality. For Westerners, typically immersed in biblical rhetoric, this was quite a new way of thinking. Enlightenment thinkers argued that man only had to discover the right keys to unlock the secrets of the universe.
You may find the following information on Newton, the Enlightenment and its principles at Washington State's website (link below). Here is an excerpt concerning Newton in particular:
The implications of Isaac Newton's physical theories of mechanics, which treated the universe as if it were a machine (hence the term "mechanics") built by a creating god yet running on its own principles independent of the interference of the creating god (though Newton never denied that God couldn't interfere, just that he didn't), encompassed much more than physical change and movement.
Soon other areas of experience came to be regarded as mechanistic and independent of divine interference: social structures, economics, politics, and so forth. Each of these areas could be understood and manipulated solely through rational methods, since they operated through consistent and orderly laws and principles