One God the Father, of all, who is above all, through all, and in you all.
When the fullness of time was come, God sent forth his Son, made of a woman.
For there is one God, and one mediator between God and men, the man Christ Jesus.
Which is how monotheist Jews like Paul saw the Father and the Son - which is obviously not the church fathers view, coming along some three-four hundred years after the apostles, by which time much Greek philosophic thought having shaped their thinking.
Note, the Trinitarian terminology of the creed makers is not there: “For there is one God in three coequal and coeternal persons.” Rather, the Father is the one God, the Son is the historical figure, born of a woman, stepping into history as a man, the man Christ Jesus.
Obviously, to Paul, the personal essence of the divine Being can only be ONE divine Person. The one divine Being God the Father incarnated Himself in human flesh.
Oneness Pentecostals believe that God eternally exists as God the Father, who is eternal, omnipresent, and invisible Spirit, John 4:24, incarnating Himself as the man Christ Jesus to save us from our sins (Matthew 1:18/Luke 1:35 /Hebrews 1:1-3).
Luke 1:35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
Matthew 1:18 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
Hebrews 1:1-3 God, who at sundry times and in divers manners spoke in time past unto the fathers by the prophets, Hath in these last days spoken unto us by his Son … Who being the brightness of his glory, and the express image of his person…
Just one God here, the Father, revealing himself in the one mediator. Hence, no one will see three separate and distinct divine people, they will only see His express image. The invisible God made visible, Jesus Christ.
The unity of the Godhead is not inconsistent with there being 3 persons. Both truths are equally taught in scripture.
You quote: "When the fullness of time was come, God sent forth his Son, made of a woman."
If you interpret this to mean that Christ was created at the moment of conception, you contradict the scripture which teaches His preexistence with the Father and eternal Godhead:
"In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by Him, and nothing was made that was made without Him." (John 1:1-3)
Joh_17:5 And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was.
IOW, this demonstrates the shallowness of the "Oneness" Pentecostals in their reading of scripture. If they argue that Christ is a created being, they are refuted here. If they argue that Christ is merely a mode or a role played by the Father, they contradict Christ's eternal presence by the side of the Father.
Which is how monotheist Jews like Paul saw the Father and the Son
All Christisn are monotheist, including Paul.
which is obviously not the church fathers view, coming along some three-four hundred years after the apostles,
False:
Ignatius, who lived in the time of John the Apostle, who lived from the first century and died either at the edge of its end, or very early in the 2nd century:
"In Christ Jesus our Lord, by whom and with whom be glory and power to the Father with the Holy Spirit for ever," (n. 7; PG 5.988).
"We have also as a Physician the Lord our God Jesus the Christ the only-begotten Son and Word, before time began, but who afterwards became also man, of Mary the virgin. For ‘the Word was made flesh.' Being incorporeal, He was in the body; being impassible, He was in a passable body; being immortal, He was in a mortal body; being life, He became subject to corruption, that He might free our souls from death and corruption, and heal them, and might restore them to health, when they were diseased with ungodliness and wicked lusts," (Alexander Roberts and James Donaldson, eds., The ante-Nicene Fathers, Grand Rapids: Eerdmans, 1975 rpt., Vol. 1, p. 52, Ephesians 7).
He calls Christ God and places Him in a Trinity of persons, the Father and Holy Spirit.
From the second century:
"The Church, though dispersed throughout the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith . . . one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations of God, and the advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord, and His manifestation from heaven in the glory of the Father ‘to gather all things in one,' and to raise up anew all flesh of the whole human race, in order that to Christ Jesus, our Lord, and God, and Savior, and King, according to the will of the invisible Father, ‘every knee should bow, of things in heaven, and things in earth, and things under the earth, and that every tongue should confess; to him, and that He should execute just judgment towards all . . . '" (Irenaeus, Against Heresies X.l)
Note that he speaks of three persons, and easily applies the title of God to Christ.
Tertullian, from the late 2nd to early 3rd century:
"We define that there are two, the Father and the Son, and three with the Holy Spirit, and this number is made by the pattern of salvation . . . [which] brings about unity in trinity, interrelating the three, the Father, the Son, and the Holy Spirit. They are three, not in dignity, but in degree, not in substance but in form, not in power but in kind. They are of one substance and power, because there is one God from whom these degrees, forms and kinds devolve in the name of Father, Son and Holy Spirit," (Adv. Prax. 23, PL 2.156-7).
And Origin, deeper into the 3rd century:
"If anyone would say that the Word of God or the Wisdom of God had a beginning, let him beware lest he direct his impiety rather against the unbegotten Father, since he denies that he was always Father, and that he has always begotten the Word, and that he always had wisdom in all previous times or ages or whatever can be imagined in priority . . . There can be no more ancient title of almighty God than that of Father, and it is through the Son that he is Father," (De Princ. 1.2., PG 11.132).
The Council of Nicea was around or a little more than a hundred years after this last quote.
Note, the Trinitarian terminology of the creed makers is not there: “For there is one God in three coequal and coeternal persons.” Rather, the Father is the one God, the Son is the historical figure, born of a woman, stepping into history as a man, the man Christ Jesus.
Is that how you interpret that sentence? It says that there are three persons, and these three persons are the same God. Therefore Christ is not a "mode" of the Father. He is eternal, never ceases, and is always by the side with the Father while possessing the same eternal substance.
God is manifest in three persons, and the doctrine of the Trinity did take about 400 years for the early Church to get down in theology. They didn’t have as much theology presented to them and it took some time for them to figure it out. We don’t have that excuse.
If anybody doesn’t understand the Trinity, keep it simple.
God manifests Himself to us in three persons, all the same one God. In order to understand His Plan for us, we simply have too understand His Word. In order to understand His Word, we read the Bible where it is presented to us.
In studying the Word, it is obvious He is manifest to all humanity in three persons, but still one God. It’s really nothing more complex than that basic understanding.
A Scriptural study of each person of the Godhead is a great way to begin to understand His Plan for us collectively and individually.
Another doctrine parallel to the doctrine of the Trinity is the Doctrine of the Hypostatic Union, which might help in discerning His role for man in relationship to God the Father.