Celtic Moment

I was finding it curious about the fact that someone mentioned that the knot on the book came from a religion that was older than Christianity. Could someone elaborate more on that? I like the TV series of Charmed, and just assumed that it was a book of spells, and that the symbol only was part of the Wicca religion. I was raised Christian, i know that is strange for me to watch something that i was strongly urged to not watch at all. Curiousity always killed the cat, or so to speak. i like the actors mainly.