Quote from ACIM Text Chapter 7, Section 9 - The Extension of the Kingdom.

The extension of God's Being is spirit's only function. Its fullness cannot be contained, any more than can the fullness of its Creator. Fullness is extension. The ego's whole thought system blocks extension, and thus blocks your only function. It therefore blocks your joy, so that you perceive yourself as unfulfilled. Unless you create you are unfulfilled, but God does not know unfulfillment and therefore you must create. You may not know your own creations, but this can no more interfere with their reality than your unawareness of your spirit can interfere with its being.