I may not be an experienced Onmyouji, but I am a Witch as well as an extremely spiritual person and therefore have much knowledge on these subjects. I will now share this knowledge with you.
Our power as Onmyoujis comes from the Divine, no matter what form it takes.
Everything at its base form is energy. Thoughts, matter, the Divine. Therefore the Divine is all and all is the Divine. We are one with the Divine, as is the energy we use to in our Magick, therefore the Magick is also one with us.
Belief and connection with the Divine, often attained through prayer and meditation, is important for our power to work. Therefore belief and connection with the Divine is also more important than knowing spells. Once we have enough knowledge we can also craft our own spells. What they say is true: Knowledge is Power.
Often the Divine takes on the form of deities, though it also takes on the form of animals, plants, people, spirits, and even so-called "inanimate" objects, as all these things are energy, as is the Divine, and therefore are one and the same.
Here is a poem from the Wiccan Tradition that briefly describes this.
I'll continue this "lesson" when I think of what else to say. 
Subject: Religion and Magick 
Sat Jun 04, 2011 10:49 am
Thee power ov thee sun!!! Thee solar deities ov thee ancient Regerie... Apollo, Ahura Mazda, THEE great Mithras!!! Shemesh.. thee solar sign ov Tipheret in thee solar plexis! Thee Dawning ov thee black sun, thee turning ov thee hakenkreuz... this is imbeded into my soul! 
RA!!! The lux Dei!!! Bathed in thee gilded splendor ov life!!!! The night, and thee day. Yin, and Yang... to stand in 2 circles, and unveil thee hidden truth in the unity ov duality.. this is my religion! Thee wisdom ov Thoth/ Hermes!!! AS above so below!! As in thee Unicursal Hexagram!!!