Since the beginning of modalism in the late second and early third century, modalists have been accusing those holding to an orthodox articulation of the doctrine of the Trinity of tritheism, the heresy that there are three gods. To this day, semi-modalists continue this classical modalistic apologetic, accusing those who hold to a classical trinitarianism, such as defined by the Nicene Creed, of being guilty of tritheism.
This of course stems somewhat naturally from the mindset of modalism. The classical modalist believes there is one God because there is only one divine person, who takes turns manifesting himself as Father, then Son, and then Spirit.
Not far different from him is the semi-modalist, who thinks that there is one God because all three real persons of the Trinity, while having some concrete distinction, are one person. Therefore when this unity of person is denied, and it is insisted that the Father, Son, and Spirit are truly distinct persons without somehow being a single person in some way as well, the semi-modalist naturally views this as leading to having three gods, since in their mind the unity of all three persons into a single person was the very reason they could confess one God at all.
Such a modalistic misunderstanding of God’s unity naturally leads a person to reject trinitarian orthodoxy as tritheism. But if the reason there is only one God, and not three, is not because the three real persons of the Trinity are in some way a single person, then what is the reason there can only be said to be one God? And if the one God of the Christian faith is the person of the Father in particular, yet the persons of His Son and Holy Spirit share His divine nature, how do they not constitute second and third Gods?
To understand the answers to these questions, it is necessary to consider what we mean when we say that there is only one God? Do we mean that there is only one divine person? The answer to this cannot be ‘yes’, since scripture clearly teaches that there are three divine persons, and yet equally teaches that there is only one God, and that this one God is the person of the Father. Because God says so in His word, we know that both of these things are true. Thus, we know that there are multiple divine persons, and that this does not constitute three Gods. Even if we do not go any further in our understanding than this, we are still required to accept these things as true, because God, Who is ultimately trustworthy, has revealed them to us. Yet even after we have accepted these things on faith, we are still left with the question of why these things are so.
When we say that there is only one God, then, we do not mean that there is only one divine person, or else we are disagreeing with the scriptures. Rather when we say there is only one God we refer to the fact that there is only one ‘head without a head’ (head=authority; head without a head= authority without a higher authority above it) and ‘uncaused Cause’. The Father alone is both of these things. The fact that the Son and Holy Spirit constitute second and third divine persons does not mean that They are second and third Gods.
The Son and Holy Spirit would constitute a plurality of gods if any of these things were true: 1) They possessed a different divine nature than the one God, or 2) They constituted Authorities equal with the Father, or 3) They were unoriginate. None of these things are in fact the case, however.
The Son and Spirit have exactly the same divine nature as the Father, as They are both from Him, the Son by eternal generation and the Spirit by eternal procession. Both of them have the Father’s divine nature communicated to Them in generation and procession respectively. There is then only one divine nature; thus no one may say that there are multiple Gods because there are multiple kinds of divinity. There is only one divine nature, that proper to the one true God, the Father, which His Son and Holy Spirit eternally participate in. Therefore, there is not a plurality of gods because of a plurality of divine natures.
The Son and Spirit also do not constitute authorities equal to the Father since He is head over them. The Father alone is the “Head without a Head”, Who Himself has supreme authority over all, with no higher authority above Him. The Son and Holy Spirit however, are under the headship of the Father. While They too possess headship over the creation, which was created through Them by the Father, They do not possess headship over the Father. So all authority ultimately runs up to one only supreme head which has no higher authority: the one God, the Father. He rules all things as a monarchy, His Son and Spirit participating in that monarchy over creation, as Those through Whom He administrates His kingdom. Therefore, there is not a plurality of gods because there is a plurality of supreme authorities.
The Son and Holy Spirit, unlike the Father, are of another, namely, the Father. The Son is of the Father by eternal generation, and the Holy Spirit by eternal procession. The Son and Spirit then are both caused by another, whereas the Father alone, the one God, is the uncaused Cause of all; not only of all creation, together with the Son and Spirit through Whom He created all things, but is even Himself the Cause of the Son and Spirit. Thus there is not a plurality of gods because of a plurality of Uncaused Causes.
So then we see that the Father being the one God is actually the foundation of biblical monotheism. It is because the Father alone is the supreme head and cause of all, and there is no other divine nature than His own, that there is only one God. There is one God because there is one Father. The divinity of the Son and Holy Spirit in no way take from the Father the quality that He is the one God; but as it is His divinity that They participate in, His monarchy They participate in, and Him from Whom They have their being, They in no way constitute a second and third God, although They are second and third divine persons, possessing the very same divinity as the Father Himself.