The fundamental problem of all forms of modalism is this: if God, in order to reveal Himself, becomes something other than what He is, then he has not revealed Himself but has revealed something else. In this case, if God emerges from a state of being a non-modal and non-interpersonal being to become a modal, interpersonal being in the story of Jesus, then He has not revealed His true non-modal, non-interpersonal self. He has revealed instead a Father-God who has interpersonal fellowship with Himself in the modal person of the incarnate God, Jesus Christ. But according to Oneness theology, that interpersonal fellowship of Father and Son is precisely the thing He is not. So the unipersonal God attempts to reveal Himself but instead reveals an interpersonal divine being. The early Christians recognized this dilemma and solved it by confessing that if God reveals Himself to us by showing Himself to have a Son, then He must always have had a Son to show us in the fullness of time. Modalists, including Oneness Pentecostals, should face the unpleasant implication that their view makes God reveal Himself as that which He is not. Such a revelation, by its nature, cannot be true.
Sanders seems to be interpreting Scripture through the lens of ecclessiology. He is at least influenced by it heavily. God is not eternally "Father" anymore than he is eternally "Son". In the Old Testament His named is revealed to us thousands of times as "YHWH". Here it must be obvious that God has various titles in His relations with His creatures.
God's title as "Father" serves to describe Him in more than one sense of Fatherhood, i.e. Creator. The Old Testament concept of Fatherhood never relates to a Trinity of persons. It relates to His created people, Israel, and is indeed the very one who brought Adam into being at the dawn of Creation.
This assumption leads to a more problematic understanding of the Trinity, at least in the Old Testament texts. For example, if the Son is eternal and was just as coequal and coeternal as the Father, in person, then His absence as such a person in the Old Testament texts is further confounding of the properties of the persons in the Trinity. In the 4th Century the Cappodocian fathers basically rendered the properties of the persons of the Trinity as beyond human reason and incomprehensible.
Fifth, I feel Sanders is making the assumption that modalism is Oneness theology. This might be a misconception and/or a failure to articulate proper theology on someone's part, but Oneness theology is not ancient modalism. It should be known that all of our excerpts or texts that relate to Oneness teachers, notably Sabellius, Noetus, or Praxeas are found in quotations by Trinitarian apologists. It should also be noted, in historical context, that many of these Trinitarian apologists weren't even bishops or held any office. In fact, Tertuallian lets us know that he discounts the "majority" who disagreed with his "trinitas" as simple, yet not unlearned. That was kind, but it was also a concession. It concedes, in some sense that Trinitarian theology is a developmental dogma that was not even widely held, but in fact seen as a violation of true Monotheism early on (See Tertuallian "Against Praxeas:) Trinitarianism has proceeded from ecclesiastical efforts. I would also say that Protestantism has not protested enough.
Much of what we read of such Oneness teachers like Sabellius or Praxeas come from the pen of Trinitarian apologists like Tertullian or Hippolytus, etc. From their accounts these modalists (a word imposed upon them most likely) all believed in one God and one person of God. It is in the discussion of nature or humanity of Christ, in some sense, that it is distinguished from Oneness theology.That is why UPC teachers should and continue to say that our theology is coming from the Scripture texts and not ancient modalism.
For example, Modalism makes Christ to be a shell or some temporal mode, i.e. pure Modalism. Some Oneness Pentecostals do speak of Jesus manifesting in temporal terms such as "skin on" or "robed in flesh" etc. I believe Oneness theology disagrees with them though. An example of Oneness theology would be by the pen of Dr. Daniel Segraves or Dr. David Bernard. Neither of them write in such modalist terms. In fact, they both defy it at times.
Ultimately, it is not easy to say that these early Oneness teachers were totally right but neither can we say that they are totally wrong either. There is common beliefs found with Sabellius and Praxes or Noetus. Due to the fact, however, that their writings are found only in Trinitarian apologetics (Tertullian, Hippolytus, etc) we do not know some very important things:
a. if what we know is not distortion or
b. if we even know everything or the full extent of their writings.
The Restoration of Christianity was translated into English in 2007. Some may know that he wrote a lengthy piece against the Trinity and was burned by fire fueled by green wood and his own books. Servetus suffered a terrible death because He would not recant his beliefs. In 1628 William Harvey would use the writings of Servetus to explain blood flow.
I believe that the Son of God was truly God and man united inseparably. The humanity and deity were united inseparably into one divine person. The Son however was subject and in perfect obedience to the Father. This does not deny His existence as a true human. God existing as man. I do not believe God assumed a human person but rather Jesus is the only expressed image of the Divine Being. The distinction of existence is metaphysical and existential. God is always the eternal God Almighty but He did come into human history. He entered into His creation. God was in Christ (2 Cor. 5:19) bringing us back to proper relationship. Jesus was the Way and the Ultimate Sacrifice. He is the visible imprint of the invisible God's being (See Hebrews 1:3 NRSV).