Here follow a few excerpts from:

The Origin of the Trinity: From Paganism to Constantine
by Cher-El L. Hagensick
 
http://www.heraldmag.org/olb/Contents/doctrine/The%20Origin%20of%20the%20Trinity.htm

If you wish to read the whole article including the references, I suggest visiting the link above.

If you prefer to read a few excerpts that highlight the main points, read on:

The words in Blue  are my own notes added for clarification.

The words in red are highlights from the text that I wanted to emphasize.

The historian, H. W. F. Saggs, explains that the Babylonian triad consisted of ‘three gods of roughly equal rank... whose inter-relationship is of the essence of their natures’

. . . 

In his Egyptian Myths, George Hart, lecturer for the British Museum and professor of ancient Egyptian heiroglyphics at the University of London, shows how Egypt also believed in a ‘transcendental, above creation, and preexisting’ one, the god

Amun. Amun was really three gods in one.

 Re was his face, Ptah his body, and Amun his hidden identity.

" Egyptian . . . three gods in one."

. . .

The well-known historian Will Durant concurs that Ra, Amon, and Ptah were ‘combined as three embodiments or aspects of one supreme and triune deity’

" . . .  three embodiments or aspects of one supreme and triune deity’ "

. . .

Additionally, a hymn to Amun written in the 14th century BC defines the Egyptian trinity:
‘All Gods are three: Amun, Re, Ptah; they have no equal.

 His name is hidden as Amun, he is Re... before men, and his body is Ptah’

. . .

Dr. Gordon Laing, retired Dean of the Humanities Department at the University of Chicago, agrees that ‘the worship of the Egyptian triad Isis, Serapis, and the child Horus’ probably accustomed the early church theologians to the idea of a triune God, and was influential ‘in the formulation of the doctrine of the Trinity as set forth in the Nicaean and Athanasian creeds’

" . . .accustomed the early church theologians to the idea of a triune God . . .

. . .

The historical lecturer, Jesse Benedict Carter, tells us of the Etruscans.

As they slowly passed from Babylon through Greece and went on to Rome, they brought with them their trinity of Tinia, Uni, and Menerva.

This trinity was a ‘new idea to the Romans,’ and yet it became so ‘typical of Rome’ that it quickly spread throughout Italy.

Even the names of the Roman trinity: Jupiter, Juno, and Minerva, reflect the ancestry.

That Christianity was not ashamed to borrow from pagan culture is amply shown by Durant: ‘Christianity did not destroy paganism; it adopted it’.

Christianity did not destroy paganism; it adopted it.
. . .

Dr. Jaroslav Pelikan, a Catholic scholar and professor at Yale, confirms the Church’s respect for pagan ideas when he states that the Apologists and other early church fathers used and cited the pagan Roman Sibylline Oracles so much that they were called ‘Sibyllists’ by the 2nd century critic, Celsus.

. . .

The attitude of the Church toward paganism is best summed up in Pope Gregory the Great’s words to a missionary:
‘You must not interfere with any traditional belief or religious observance that can be harmonized with Christianity’

‘You must not interfere with any traditional belief or religious observance that can be harmonized with Christianity’

. . .

Even though ‘Word,’ ‘Spirit,’ ‘Presence,’ and ‘Wisdom’ are used as personifications of God, Biblical scholars agree that the Trinity is neither mentioned nor intended by the authors of the Old Testament

. . .

As for his relationship with the Father, Jesus said,
 ‘... I seek not mine own will, but the will of the Father which hath sent me’,  {# Joh 5:30}
...‘my doctrine is not mine, but His that sent me’;   {# Joh 7:16}

. . .

The word ‘trinity’ was not coined until Tertullian, more than 100 years after Christ’s death,
and the key words (meaning substance) from the Nicene debate, homousis and ousis,  are not biblical, but from Stoic thought.

A person named Tertulian was the first Christian theologean to use the word "trinity" to describe God.
He used that term about a hundred years after Jesus left the earth.
And yet Tertulian's concept of the trinity did not match exactly with the concept of the trinity that the Church adopted later.

Nowhere in the Bible is the Trinity mentioned.

Nowhere in the Bible does the word "trinity" appear, anywhere. That is what the author, Hagensick, meant.

According to Pelikan, ‘One of the most widely accepted conclusions of the 19th century history of dogma was the thesis that the dogma of the Trinity was not an explicit doctrine of the New Testament, still less of the Old Testament, but had evolved from New Testament times to the 4th century.


If the Trinity did not originate with the Bible, where did it come from?

To find the origins of the Trinity in Christianity, we need to take a look at the circumstances in which early Christians found themselves.

 
Even the Church of the Apostles’ day was far from unified.

. . .

According to McGiffert, the concepts of philosophy prevalent during the time of the early church were Stoicism, . . . and Platonism . . .

. . .

That these philosophies affected Christianity is a historical fact.

What did these philosophers teach about God?

In Plato’s Timeus, ‘The Supreme Reality appears in the trinitarian form of the Good, the Intelligence, and the World-Soul’

. . .

Laing attributes elaborate trinitarian theories to the Neoplatonists, and considers Neoplatonic ideas as ‘one of the operative factors in the development of Christian theology’

. . .

Durant ties in philosophy with Christianity when he states that the second century Alexandrian Church, from which both Clement and Origen came, ‘wedded Christianity to Greek philosophy’

. . . and finally, Durant writes of the famed pagan philosopher, Plotinus, that ‘Christianity accepted nearly every line of him...’

. . .

In the desire to grow, the Church compromised truth, which resulted in confusion as pagans became Christians and intermingled beliefs and traditions.

. . .

As more and more pagans came into Christianity, they found the Judaic influence offensive.

. . .

Harnack affirms that the early church view of Jesus was as Messiah, and after his resurrection he was ‘raised to the right hand of God’ but not considered as God .

" . . . the early church view of Jesus was as Messiah . . . but not considered as God."

. . .

As for the holy Spirit, McGiffert tells us that early Christians considered the holy Spirit ‘not as an individual being or person but simply as the divine power working in the world and particularly in the church’.

" . . . early Christians considered the holy Spirit ‘not as an individual being or person but simply as the divine power . . . "

. . .

Durant summarizes early Christianity thus: ‘In Christ and Peter, Christianity was Jewish; in Paul it became half Greek; in Catholicism it became half Roman’

in Paul it became half Greek

. . .

The world around the early Church was changing.

The Roman empire began to crumble and Constantine came to power.

He wished to unify the Empire, and chose Christianity to do so.

But Christianity was far from unified.

. . .

He concludes that the Nicene dogma marked the ‘transition from the prophetic Oracle of Yahweh... to Catholic dogma’

. . .

The Nicene was not a popular creed when it was signed.

Durant affirms that the majority of Eastern bishops sided with Arius in that they believed Christ was the Son of God ‘neither consubstantial nor co-eternal’ with his Father.

Arius was a priest who had many followers.
His followers were known as Arians (not to be confused with the ethnicity, Aryan.
His views of Jesus more closely matched those of Islam.
The last of the Arians were slaughtered just before Muhammad began to receive his revelations.
So, we see that when the truth of Jesus was destroyed from the world, God Al-Mighty sent anothre prophet to set the record straight.


. . .

There is an unfortunate side to the whole Athanasian/Arian debate.

Campbell could find no parallel in medieval nor modern times in the intensity of debate.

The debate between the Arians, who had a view of Jesus and a view of God which was similar to Islam, and the followers of Athunasias was so heated that it often led to extreme violence.
In the end, it was the followers of Athunasias who won.


Durant details the problems that arose from the Council at Nicea and summarizes that period with a dreadful verdict:
‘Probably more Christians were slaughtered by Christians in these two years than by all the persecutions of Christians by pagans in the history of Rome’

. . .

The evolution of the Trinity can be well seen in the words of the Apostles’ Creed, Nicene Creed, and the Athanasian Creed.

As each of the creeds became more wordy and convoluted, the simple, pure faith of the Apostolic church became lost in a haze.

" . . . the creeds became wordy and convoluted . . . "

Even more interesting is the fact that as the creeds became more specific (and less scriptural) the adherence to them became stricter, and the penalty for disbelief harsher.


In summary, the common culture of the day was one filled with triune gods.

From ancient Sumeria’s
- Anu, Enlil, and Enki

and Egypt’s dual trinities of
- Amun-Re-Ptah
and
- Isis, Osiris, and Horus

to Rome’s
- Jupiter, Juno, and Minerva

the whole concept of paganism revolved around the magic number of three.

In Greek philosophy, also, we have seen how the number three was used as an unspecified trinity of intelligence, mind, and reason.

Constantine invited the bishops from East and West to join him in the small seaside village of Nicea for a council to unify the church.

McGiffert summarizes the council: three main groups were present at this council:
Eusebius of Nicomedia presenting the Arian view of the Trinity,
Alexander of Alexandria presenting the Athanasian version,
and a very large ‘middle party’ led by Eusebius of Cesarea whose various theological opinions did not interfere with their desire for peace.

Eusebius of Nicomedia submitted the Arian creed first and it was rejected.

Then Eusebius of Cesarea submitted the Cesarean baptismal creed.


Instead of submitting a creed of their own, the anti-Arians modified Eusebius’, thereby compelling him to sign it and completely shutting the Arians out.

Those Arians who did not sign were deposed and exiled


Thus Constantine had his unified Church which was not very unified.

McGiffert asserts that Eusebius of Cesarea was not altogether satisfied with the creed because it was too close to Sabellianism (Father, Son, and Holy Spirit are three aspects of one God).

Eusebius was uncomfortable enough with the Nicene creed that he felt it expedient to justify himself to his own people in a long letter in which he states that he ‘resisted even to the last minute’ until the words were examined and it was explained that the words ‘did not mean all they seemed to mean but were intended simply to assert the real deity of the Son...’.

McGiffert goes on to show that a ‘double interpretation [was authorized by the leaders in order to win Eusebius and his followers.’.
 
Lonergan shows just how much of the creed Eusebius took exception to as the words were explained.

‘Out of the Father’s substance’ was now interpreted to show that the Son is ‘out of the Father’, but ‘not part of the Father’s substance.’

‘Born not made’ because ‘made’ refers to all other creatures ‘which come into being through the Son’, and ‘consubstantial’ really means that the Son comes out of the Father and is like him.

It is clear that the council strongly lacked unity of thought.


Lonergan goes on to explain that the language of debate on the consubstantiality of the Father and the Son has made many people think that the ‘Church at Nicea had abandoned the genuine Christian doctrine, which was religious through and through, in order to embrace some sort of hellenistic ontology’.

The 19th century scholar and Protestant minister, Alexander Hislop, devotes several chapters of his book The Two Babylons to showing how this original belief in one god was replaced by the triads of paganism which were eventually absorbed into Catholic Church dogmas.

. . .

Hislop devotes the first 128 pages of his book The Two Babylons to proving that the Christian Trinity is directly descended from the ancient Babylonian trinity.

 In particular, he convincingly proves that the origin of the Babylonian trinity was the triad of Cush (the grandson of Noah), Semiramis (his wife), and Nimrod (their son).

At the death of Cush, Semiramis married her son, Nimrod, and thus began the confusion between the father and son so prevalent in early paganism.

. . . and thus began the confusion between the father and son so prevalent in early paganism.