If God who is not the Father, then there is a God who is not God the Father
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic God
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“Therefore it is false to say, "God begot God." Obj. 5: Further, if "God begot God," He begot either God who is the Father, or God who is not the Father. If God who is the Father, then God the Father was begotten. If God who is not the Father, then there is a God who is not God the Father: which is false. Therefore it cannot be said that "God begot God." On the contrary, In the Creed it is said, "God of God." I answer that, Some have said that this name "God" and the like, properly according to their nature, stand for the essence, but by reason of some notional adjunct are made to stand for the Person.” source