The Book And The Brotherhood - pg. 488

He sang both high and low. He promised strength through repentance, and joy through renewal of life. He exhorted her to remake herself into an instrument fir for the service of others. He used the oldest argument in the book (sometimes called the Ontological Proof) which, in Father McAlister's version, said that if with a pure passion you love God, then God exists, because He has to. After all, what your best self, your most truthful soul desires must be real, and not to worry too much about what it's called.