In John 5:43-44 Jesus says to his fellow Jews “I have come in my Father’s name [and] you don’t seek the glory that comes from the one Who alone is God.” (NLT)

In John 8:54 he adds “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.”

Then in John 17:3 Jesus, praying to his Father, identifies Him as the only one Who is true God. And in John 20 Jesus says to Mary that he is “ascending to my Father and your Father, to my God and your God.”

Even in heaven the Son calls his Father “My God” 4x alone in Rev 3.12! 

This same understanding of the Father as the one Who alone is true God was shared by the Apostles. Paul greets the churches this way:

“Grace to you and peace from God our Father and the Lord Jesus Christ.”

1Cor 1.3; also Phil 1.2; Rom 1.7; 1Thess 1.1, etc.

In Eph. 4.6 Paul states there is “one God and Father of all, who is over all and through all and in all.”

In 1Cor 8:6a Paul says that Christians have “one God, the Father.”

“The fact of the matter is that Paul does not say that there is one God who is both Father and Son; he says rather that there is one God.”[1]

Jesus and his Apostles inherited the understanding the their one God is the Father from the OT scriptures, where He is known as Yahweh or Jehovah: Deut 32:6; Mal 2.10; also Jer. 3.19; 31.9; Isa 63.16; 1Chron 29.10; Mal 1.6; Pro 3.12; Ps 103.13.

Many trinitarian publications and scholars admit the same thing.

Encyclopedia Brittanica: “Believers in God as a single person (God the Father), were at the beginning of the 3rd century still forming the large majority.”

And Dr. Murray J. Harris (in his often cited book Jesus as God) notes that “in all strands of the NT, theos generally signifies the Father. When we find the expression theos pater we may legitimately deduce that o theos estin o pater. And since pater refers to a particular person (not an attribute), the identity between o theos and o pater as proper names referring to persons must be numerical: God is to be equated with the Father.”

[1] James McGrath, The Only True God, pp. 38-43.