in reference to Dan Brown's Angel and Demons

"Illuminati, Greek illumination, name given to those who submitted to Christian baptism. Those who were baptized were called "illuminati" or "illuminated ones" by the Ante-Nicene clergy, on the assumption that those who were instructed for baptism in the Apostolic faith had an enlightened understanding.

The Alumbrados, a mystical 16th-century Spanish sect, were among the societies that subsequently adopted the name illuminati. Later, the title of illuminati was used by a secret society founded by Adam Weishaupt that aimed to combat religious thinking and encourage rationalism."
                                                         ---Microsoft Encarta2000

from the source....Masonic Info

http://www.masonicinfo.com/illuminati.htm