Why modern day modalist doctrine rejects God's oneness

Divine Simplicity is a doctrine often times brought up to challenge the orthodox doctrine of the Trinity which is believed by all Christians and denied by non-Christians. I go so far to say that because, simply put, without the Trinity, there is no salvation. Not that invincible ignorance might lead to damnation. I cannot make judgments on that as I am not God, but that unless God is Triune, the doctrine of salvation as taught by the Church is incoherent. Christ's Mission on Earth was to defeat Death and Hell and He did exactly that. To those denying that He is Divine in and of Himself, they would posit that a mere mortal could do such. And to those confounding the Persons of the Trinity, well...

A large problem is there is ample literature on Trinitarian doctrine and Trinitarian apologetics and the Church's classical doctrine and teaching are often times buried in the philosophical mumbo-jumbo that modern day anti-Trinitarians accuse Trinitarians of holding. The philosophical mumbo-jumbo about the Trinity in modern day Trinitarian apologetics is rarely seen or observed in classical orthodox dogmatics. That is because that mumbo-jumbo never even occurred to the Church. I see a lot of anti-Trinitarians using the word "Godhead" to refer to the Trinity as if "Godhead" is the same as God or even the famed "Trinity Delusion" website. They mostly respond to the anti-cult hunters and the Trinitarian apologists who are divorced from classical Christian doctrine.

The Trinity Delusion website is a class example. In the article I linked, it enforces modern understanding of the terminology upheld at Nicaea to arrive at the conclusion that the "nature" cannot be a "Who" but a "What". This is echoed strongly among Trinitarian apologists and lends itself to the greatest anti-Trinitarian strawman attack ever. I used to be an anti-Trinitarian and that was the dogma I attacked. But it was not the doctrine taught in historical Christian theology. In fact, in order to understand what Trinitarians mean by "ousia" and "persons" and "beings", a knowledge of the historical controversies has to be gathered. In Cardinal Joseph Ratzinger's Introduction to Christianity, he has an entire chapter dedicated to proving the point that the Trinitarian doctrine is built on a graveyard of heresies. It was Sabellians who first used terms such as "ousia" and "persons" and "beings" and this was why the Church was reluctant at first to adopt Nicene orthodoxy.

And that brings us to Sabellians of the modern day who are mostly found among a group called "Oneness Pentecostals". When contending with an upholder of this modern day Oneness philosophy, a variant of modalism which insists that Jesus is the Father, the Son, and the Holy Spirit, much appeal is made to the hypostatic union doctrine. Of course, it is not. Sabellians will say that their doctrine is consistent with Divine Simplicity because they misunderstand the proper Trinitarian theology regarding the Father, the Son, and the Holy Spirit. The Trinity is not a "group" deity nor is the Trinity a conglomeration of "parts" and the Father, the Son, and the Holy Spirit are not parts of God either. Rather the Father is wholly God, the Son is wholly God, and the Holy Spirit is wholly God. If God was divided into parts, this would violate Divine Simplicity. But God's Oneness is found in His infinitude. Infinity is the only thing which cannot be divided into parts. God is infinite in nature, indivisible in nature, hence, the three persons of the Trinity can never be acknowledged as divided or in parts.

But when confronted, what you will notice with "Oneness theologians" is that they frequently separate the Son from the Father. How else will they get two witnesses (John 8)? How else do they explain the baptism of Christ or the Transfiguration? Either the Son and the Father are two persons or the Son and the Father are two entities which are not united as one together. The Son would have to be a projection created by the Father. This exceeds the hypostatic union doctrine of Chalcedon. While the hypostatic union differentiated between a human nature and divine nature had by Christ, it never denied that Christ was wholly talking as God. There are many places in Scriptures where Christ conceals His deity, but He never denies it. Matthew 24:36 is often times brought up by anti-Trinitarians on both sides and the Church has never accepted the interpretation that it marks Christ as "ingorant". There are many senses of "knowing". In knowing the day and hour, Christ does, but it is not to the benefit of His Mission. He is fully aware of the events that shall lead up to that hour and so He does know the hour. What is not of His earthly Mission is to judge the world. St. Augustine writes: "That He says that the “Father knoweth,” implies that in the Father the Son also knows." (Serm. 97, 1)

Further, St. Hilary of Poitiers elaborates on the text by indicating that "in all cases, in which God declares Himself ignorant, He is not under the power of ignorance, but either it is not a fit time for speaking, or it is an economy of not acting." (On the Trinity, IX) Therefore, we see that it is in the humanity, of being contained in finitude and time, that the Son is not here eternally acting, and therefore confesses not knowing. For is in such that He is not at act that He states His ignorance in figurative language. For both the Arians and the Modalists, the omniscience of the Son is denied outright by this text. And the Modalists have such a perverse view that they will proceed to differentiate the man Jesus Christ from God. But if Jesus Christ is the Father in their theology, then Who was incarnated? And that is where the Modalist position collapses. In essence, in denying that the Son is the Father and yet insisting that Jesus is the Father and the Son, the modern day Modalist or Oneness position gives itself over to philosophical reasonings in a desperate attempt to preserve it's anti-Scriptural theology. And it splits God into two - a man and a god. Oneness doctrine therefore cannot uphold in any matter the doctrine of Divine Simplicity. For their "Oneness" of God is a Jesus that is split into the Son and the Father who are both Jesus but not each other, meaning Jesus has a conversation with Himself, declares Himself His own God, prays to Himself, declares Himself to be His own Son, etc.

Triune Oneness posits the infinitude of God which cannot be divided. The Trinity is not merely a "Godhead". The Trinity is God. And the Father is God. The Son is God. The Holy Spirit is God. The Father is the only true God. The Son is the only true God. The Holy Spirit is the only true God. The Trinity is the only true God. Those statements cannot all be true unless they are wholly, uniquely, God, in and of themselves, and are indivisible. The "Oneness" deity is divided against himself.