Origen (3rd-century "Church Father") wrote regarding John 1:1:
We next notice John's use of the article in these sentences... He uses the article when the name of God refers to the uncreated cause of all things, and omits it when the Logos is named God...as the God who is over all is God with the article, not without it.
God on the one hand is Very God (Auththeos, God of Himself); and so the Savior says in His prayer to the Father [John 17:3], "That they may know the only true God"; but that all beyond the Very God is made God by participation in His divinity, and is not to be called simply God (with the article) but rather God (without article).
Ante-Nicene Fathers Vol. 9, p. 323