“Say, [O believers], “We have believed in Allah (the Creator) and what has been revealed to us and what has been revealed to Abraham and Ishmael and Isaac and Jacob and the Descendants and what was given to Moses and Jesus and what was given to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [in submission] to Him.” (Quran 2:136)
The belief in the Oneness of God – as a theological movement – began at a very early stage in the history, and in fact it preceded the belief in trinity by many decades,  and never was found in the teachings of any of the prophets of God i.e. Abraham, Moses even Jesus Christ himself.
All Prophets carried the same message to all nations. A simple straightforward message as the condition of salvation: Belief in One God (the Creator) and unifying Him in worship. Every Prophet was the way for his followers in his time to get the salvation, and that was by following his teachings; worshiping like the Prophet does, not worshipping the Prophet himself or any other intermediary (idol, saint, priest, etc.), as it is the right of the Creator to be worshipped alone and the right of the human being to have direct connection with His Creator.
Prophet Abraham is the ancestor of Judah, Judah whom the Jews came from, is a descendent of Prophet Abraham, and the Jews firmly believes that there is only one God. Judah taught his people the religion of Prophet Abraham and the religion of all the Prophets who came before him which is Pure Monotheism (believing in One God and unifying Him in worship). Which is The exact definition of the religion of Islam, the religion which started from Prophet Adam and ended with the coming of Prophet Muhammad.
Prophet Jesus was also one of those pious messengers who endeavored to guide their people to the truth, but many people followed their desires and thus went far from the prophetic teachings. Jesus came to confirm the message of Moses.
- “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. ” (Mathew 5:17)
It’s difficult to reconcile the original message of Jesus, the honored human prophet, preaching in Jerusalem to the Trinitarian Christianity with Jesus as God and/or son of God adopted in the Council of Nicea three centuries later.
- “The most important one, answered Jesus, is this: ‘Hear, O Israel: The Lord our God, the Lord is One.Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’” (Mark 12:29)
- “Now this is eternal life: that they know you, the only True God, and Jesus Christ whom you have sent.” (John 17:3)
The doctrine of the trinity which was affirmed in the fourth century CE bears no resemblance to the original teachings of Christ concerning the nature of God. It is a complete deviation from his teachings. Hence it developed in opposition to the belief in One God.
Will Durant says:
“When Christians conquered Rome, the new religion (i.e., Christianity) was infused with the blood of the old idolatrous religion. It came into Christianity as the blood of the mother comes into her child. The civilized empire handed over power and administration to the papacy and the impact of the word replaced the impact of the sword. The preachers of the church started to have positions of power. Christianity instead of putting an end to idolatry, it reinforced it. The Greek faith came back in the rituals and the doctrines of the church and the monastic saints.”
The doctrine of the holy trinity, the idea of worshipping the mother and the child and The idea of the mystical union with God came from Egypt, and led to Platonism, agnosticism, and the erasing of Christian doctrine.
Mithraism, which is a religion of Persian origin, prospered in Persia around six centuries before the birth of Christ, and it reached Rome around the year 70 CE, where it spread throughout the Roman lands. Then it reached Britain and spread to a number of British cities.
Mithras, was an intermediary between God and man (a similar doctrine in Christianity).
- He was born in a cave or in a corner of the earth.
- His birthday was December 25 (which is the day celebrated by the Christians as the day when Jesus was born)
- He had twelve disciples.
- He died to save the world.
- He was buried but he came back to life.
- He was called ‘Savior’.
- Among his attributes is that he is like a peaceful lamb.
- The ‘Divine supper’ was held in his memory every year.
- One of his symbols was baptism.
- Sunday was sacred to them.
Leon Joteh  says:
“The origin of the concept of trinity was found in the Greek philosophy, specifically in the ideas of modern Platonism, which took the basis of the idea of trinity as a view of the Creator of the universe from Plato, then developed it to a great extent, so that the resemblance between this idea and Christianity became greater. So (in their view) the Creator, the One Who is absolutely perfect, appointed two intermediaries between him and mankind, who emanated from Him, and were also part of Him at the same time, meaning that they are contained in His essence. These two entities are reasoning and divine spirit.”
Then he said:
“The marriage of Jewish belief and Greek philosophy did not only produce philosophy, rather it produced a religion too, namely Christianity which imbibed many ideas from the Greeks. The Christian concept of divinity is taken from the same source as modern Platonism. Hence you see many similarities between the two, although they may vary in some details. They are both based on a belief in trinity, in which the three ‘persons’ are one.”
“..,and the Christians say, ‘The Messiah is the son of God.’ That is their statement from their mouths; they imitate the saying of those who disbelieved [before them]., ” (Quran 9:30)
Draper  says:
and polytheism entered Christianity through the influence of the hypocrites who
occupied positions of influence and high positions in the Roman state by
pretending to be Christians, but they never cared about religion and were not
sincere at all. Similarly, Constantine had spent his life in darkness and evil,
and he did not follow the commands of the church except for a short while at
the end of his life.”
 American Encyclopedia.
 An American writer.
A French Orientalist ‘An Introduction to Islamic philosophy’
 An American Writer.
The Council of Nicaea, the first ecumenical debate held by the early Christian church, concludes with the establishment of the doctrine of the Holy Trinity, convened by Roman Emperor Constantine I in AD 325.