Why not instead ask for a quote from the Gathas that says that Ahura is separate from The World?
Because there is none.
The option did not exist at the time of Zarathushtra. Among Indo-Europeans 3,700 years ago the idea of God as something separate from Existence did not exist. There as not even a concept of God. There are the concepts of Ahura (that which has existance, including everything that has existance) and the concept of Mazda (that which has mind).
And that which is taken for granted will never be quoted in any text.
Or rather, the only quote you need is a single word: Ahura! The meaning of which is the quote you ask for.
In your reply to Arthur regarding Megatheism you wrote: "as Zoroastrians we believe all that exists is a part of God."
Perhaps you or others in this discussion group can assist me. I have been looking for a direct Avestan quote (if one exists) that states words to the effect that "all that exists is a part of God." If you or others are aware of such a quote, I will greatly appreciate the reference. If the reference is an English translation, I will appreciate the translation and the name of the translator.
P.S. Just to clarify, at this stage I'm not looking for opnions or commentary - just a direct quote or translation to assist me in my research. A separate email follows requesting opinions.