A potential misunderstanding of the matter easily leads a person to think at first glance that teaching the doctrine that the one God is the Father somehow undermines the doctrinal truth of the Son’s full divinity. While I hope to show below that this is absolutely not the case, I first want to acknowledge that the objection can be brought up with a certain amount of apparent validity.
Many heretical sects teach that the Father is the one God to the denial of the Son’s divinity. The most notorious of these would be Arians and others who follow variations of Arius’s teachings, such as modern JWs. The various sects of the Jews and Muslims also tend to make use of passages in the scriptures that teach the identicality of the one God with the Father to argue against the fact that the scriptures teach the divinity of the Jesus Christ. These false teachers’ misuse of the biblical data to argue against the truth gives an understandable reason for some to be hesitant when they hear a trinitarian speaking the same way.
But we must also remember that it is the ordinary tactic of false teachers to blend their errors with truth, as one might hide poison in something sweet, in order to make their error seem palatable. There is a danger then to be acknowledged in disregarding everything that a given heretic believes as automatically being false. Were we to take this to an extreme, in most cases we would even be required to reject the scriptures because the heretics make use of them in addition to true Christians.
We must not, then, throw out the baby with the bathwater. The truth that the Father is the one God does not in truth undermine the doctrine of the Son’s divinity at all: rather, when properly understood, it implies it and serves as a doctrinal support for it.
That reasoning goes something like this: The Father is the one God. Therefore, the Son is the Son of the one God. Therefore, the Son must be God in nature just as the Father is, eternally of the same divine nature as He.
Now allow me to break that down a little more. Here we are dealing with the historic trinitarian doctrine of the eternal generation of the Son, the implications of which are enormous. In plain English it is the doctrine that Christ is the Son of God, not in name only, nor as a figurative “Son”, nor because He has been adopted by the Father, but because He was begotten of the Father before creation. This is what scripture is referring to when it speaks of Jesus Christ as the “only-begotten” Son of God. Its telling you what kind of Son Jesus is: an only-begotten one.
That stands in contrast to us as creatures. Believers are indeed called children of God by the scriptures, which reveal Him as our adoptive Father. But we remain of a creaturely, human nature. We aren’t somehow of the same divine nature as God because we are adopted as God’s children. But Christ isn’t adopted; He is Son because He was begotten by God, before and outside of time (this should remove all thought of chronological sequence from our understanding of this event). The fact that He is begotten logically necessitates that we understand Christ to be of exactly the same divine nature as the Father.
To prove this point, let us merely consider fatherhood in creation, which scripture tells us is modeled after God’s own fatherhood of the Son (Eph 3:14-15). Everything begets after its own kind. Birds beget birds. Men beget men. There is always necessarily an identically of nature between that which is begotten and that which begets. “What is begotten of spirit is spirit and what is begotten of flesh is flesh” Jesus taught in John 3:6.
This concept has historically been referred to by theologians using the philosophical language of “consubstantiality”, or “co-essentiality”, which each mean the same thing. The Greek word behind this term, “homoousias”, was used by the Nicene Council in the Creed they composed to indicate the exact identically and sameness of the divine nature of the Son with that of the Father (Who they call the one God). They grounded that teaching in the eternal generation of the Son:
“And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before the ages; God of God, Light of Light, very God of very God; begotten, not made, being of the same essence [homoousias] as the Father, by whom all things were made.”
This doctrine of eternal generation is crucial to rightly understanding the doctrine of the Trinity. This is in large part because it teaches us that if the Son is truly begotten of the Father, He must necessarily be of the same divine nature as Him. This is certain proof for the Son’s eternal Godhood. So let’s bring this back to the one God being the Father: according to this paradigm, the Son being the Son of the one God, by means of eternal generation, logically necessitates that we understand the Son to be of the exact same divine nature as the one God Who begat Him.
This doesn’t then undermine the Son’s divinity at all. In fact if the one God is eternally Father, which He is, then He must eternally have a Son. And the fact that this Son was eternally begotten of Him proves that He is eternally of the very same divine nature. The real question to ask the one who believes that the one God is the Father and yet denies the divinity of the Son is this: in what sense do you really believe that the one God is Father? In name only? Or is He really, eternally Father because He begat the Son before the ages, as the Nicene Creed teaches? If this is so (and it is), then His Son must necessarily be divine, of no other nature than that of the one God Who begat Him. The fact is, you cannot reasonably believe that the one God is the Father without also believing the full and eternal divinity of the Son as well.