The Father is called “the one God” or “the only God” at least 30 times by John in his Gospel!
For example, in the classic, clear words in John 17 John records the Son praying to the Father as the Only One Who is True God! Here, the Son clearly teaches that the Father is the only Person in the category of “true God.”
In other words, Jesus is not just saying that there is “one God” only, after all even Mormons and Muslims would agree with that statement, but that the Father is “the only true God.”
That means it’s impossible for anyone else to also be named as “the only true God.” In fact, it would be nonsensical and self-contradictory to say anyone else is also “the only true” of anything!
This teaching is so devastating to Orthodox Christianity that early so-called “Church Fathers” like Augustine, Aquinas, Ambrose and several others changed the text of John 17:3 to read:
“You [i.e., Father] and Jesus Christ….the only true God.”
This is such a blatant abuse of scripture that even some noted Protestant scholars like German Heinrich Meyer calls this reading “a perversion of the passage, and running counter to the strict monotheism of John. Only One, the Father, can absolutely be termed the only true God [and] not at the same time Christ (who is not even in 1 John 5:20 the only true God).”
This teaching is made clear early on in John 5 where the word God = the Father 14 times alone! In vv.43-44 Jesus points back to his shared Jewish, unitarian monotheistic belief when he says:
“I have come in my Father’s name [and] you don’t seek the glory that comes from the only God.”
In John 7-8 the word God is identified as the Father more than 10 times! Where Jesus echoes the words of John 17:3 when he says “the one Who sent me [i.e., commissioned] is true” (v.26).
In John 8:54 he says to his fellow Jews:
“If I were to honor myself, that honor would be worth nothing. The one who honors me is my Father—the very one you say is your God.”
Another Evangelical scholar, Marianne Thompson, notes that “throughout this argument the author of the Gospel assumes that Jesus and his adversaries [i.e., his fellow Jews] are talking about the same God”!
In John 20 Jesus says to Mary:
“Go to my brethren and tell them: I am going to my Father and your Father, to my God and your God.”
Here, Jesus clearly has a God.
If you are said to have a God you cannot also be that same one God!