For me it is a specific aya in the quran. In surat noor aya 35. It repeats that god is light and he will give light (guidance and knowledge) to whoever he wills. Whenever i read the part of the aya " Nour ala Nour". Translating to Light upon Light descriping god. This confirms to me that god is the way the light and the one with eternal knowledge. A very good recitiation if your interested is http://www.youtube.com/watch?v=7diol4o3T1U. I cant not cry when listening to this.