Several problems with
that part of your post.
The current mainstream teaching in Christianity is that God is a coequal, coeternal, one-substance trinity, and that Jesus Christ is God.
There is one vital fact you omit: according to the Trinitarian, Nicene faith, Jesus Christ is also man. He is consubstantial with us according to hypostatic union.
This doctrine is considered by many as the cornerstone of Christianity,
By nearly all, not "many." Unitarians, neo-Arians, Modalist Oneness Pentecostals and others are but a tiny minority.
but where did this doctrine come from? The historical record is overwhelming that the church of the first three centuries did not worship God as a coequal, coeternal, consubstantial, one-substance three in one mysterious godhead.
Not true. The Gospels of Ss. Matthew and John aside, we see the Nicene position anticipated in Tertullian, Origen, and St. Irenaeus of Lyons, among other places.
The early church worshipped one God and believed in a subordinate Son.
If by "created," this is untrue; we first see this among the heretical Ebionites, and later in the 3rd century heresiarch Paul of Samosata and his disciple Lucien, the mentor of Arius.
The trinity originated with Babylon, and was passed on to most of the world's religions.
I have no idea what tract you pulled that from; it is quite false, as the Babylonian pagan religion had seven major deities. Chaldean astrology linked these to the seven planets, and created an elaborate system of divinization based on them, which survives as the contemporary western Horoscope, in Mandaeism, and also as an influence in the Yazidi and Yarsani religions.
This polytheistic (believing in more than one god) trinitarianism was intertwined with Greek religion and philosophy and slowly worked its way into Christian thought and creeds some 300 years after Christ.
Not true. Arius on the other hand sought to redefine "Logos" according to the neo-Platonic idea of what the Logos would be, which we see developing in, for example, Philo.
The idea of "God the Son" is Babylonian paganism and mythology that was grafted into Christianity.
I have addressed the false accusation of a Babylonian origin for this doctrine; I shall now address the charge of mythology. No elements of Babylonian mythology were added to Christianity. Some Jews, particularly Karaites, argue that the idea of the devil is borrowed from Zoroastrianism, but this is untrue, in that Angra Mainyu represents a rival malevolent deity in ancient Zoroastrianism (modern Zoroastrianism has clipped his wings a bit by borrowing a Christian conception of diabology).
The only commonality between the Christian faith and the Chaldean religion on the other hand is in the form of certain aspects of the Old Testament, loke the idea of a flood, which are common throughout Semitic religion and folklore.
Worshipping "God the Son" is idolatry, and idolatry is Biblically condemned; it breaks the first great commandment of God of not having any gods before him (Exodus 20:3).
This would be true if we were tritheists in the manner of some Eutychians, but we are not; our Lord and the Holy Spirit are the same God as the Father.
Then three centuries after Christ the corrupt emperor Constantine forced the minority opinion of the trinity upon the council of Nicea.
Nearly all participants at Nicea supported the Nicene creed. What actually happened:
Arius was a priest in the Church of Alexandria. He began to stress the idea that Jesus Christ was a creature, which the patriarch, St. Alexander of Alexandria, objected to. Alexander sought to depose Arius, but Arius was politically adroit, and had supporters, such as the cunning Eusebius of Nicomedia, wno later baptized Emperor Constantine before his death.
Thus, a schism resulted, and the Emperor convened the council as a neutral party in order to restore peace to the church. At the council, the protodeacon of St. Alexander, St. Athanasius of Alexandria, led the charge against the Arian heresy, and was later severely persecuted by Arians, spending much of his life in exile.
The Christian church went downward from there; in fact some of the creeds and councils actually contradict each other.
Not true, for reasons we will explain:
the council of Ephesus 431 said that "human beings are totally depraved,"
Ephesus does not teach that, rather, what you refer to is a Calvinist interpretation of the Augustinian criticism of the heresy of Pelagius, which was another matter. Rather, Ephesus condemned Nestorianism, the idea that the humanity and divinity of our Lord are separate, in a personal union or union of will, as opposed to hypostatically united. Most Christians, including those of my own church, do not believe in total depravity; this is a Calvinist doctrine which is roughly a mirror image of Pelagianism (Pelagius taught that we save ourselves, Calvin taught that we are saved purely by God wirhour any action on our part contributing to it).
The Calvinist form of monergism had been condemned in effect 900 years or so before Calvin, at the Sixth Ecumenical Council.
the council of Chalcedon 451 said that "Jesus Christ is both man and God."
Not true; Nicea said that as early as 325. Rather, Chalcedon was the process of a schism between the Western, Eastern and Oriental churches over how the idea of hypostatic union was to be expressed. There was one legitimate heretic of the Chalcedonian era who was condemned by both sides of the schism: Eutyches, who taught that the humanity of our Lord dissolved into His divinity, a view similiar to the earlier heresy of Apollinarianism and the later heresy of Monothelitism.
If you follow the logic here then first you have Jesus Christ as God, then you have man totally depraved, and then you have Jesus Christ as man and God. If Jesus Christ is both man and God does this mean that God is also totally depraved?
This might be a legitimate criticism of Calvinism.
However, if we step back from your erroneous invocation of total depavity, to the idea of original sin as believed in by most Christians (Orthodox, Catholic, et al), it has never been taught that our Lord was depraved. Rather, He took onto himself our fallen nature, and restored and glorified it, facilitating our own salvation.
Well maybe the doctrine of the coequal, coeternal, one-substance, mysterious three in one triune godhead is deprived of any historical foundation tying it into the Christianity of the Bible and the Christianity of the first three centuries.
Not true; we encounter this doctrine in the Gospels.
However the historical information ties the trinity into various pagan origins.
Not true; the Trinitarian doctrine is entirely unlike any other religion, although later some portions of Hinduism sought to resolve an internal schism by borrowing the concept in part (see Trimurti, which postdates Nicea and the adoption of the Nicene Creed by the Syriac Orthodox Christians of India by several hundred years).