How did Christianity become mixed with polytheistic beliefs?
Question

                If true Christianity brought the message of the Oneness of God (Tawheed), and stated that He alone is to be worshipped to the exclusion of anyone else in His creation, whether that is ‘Eesa (Jesus) or anyone else, then how did this religion become mixed with polytheistic beliefs (shirk) and how did they take ‘Eesa (Jesus – peace be upon him) and his mother as gods besides Allaah?.
Praise be to Allah.
      There can be no doubt that the call to believe in Allaah 
      alone (Tawheed) and to worship Him alone to the exclusion of anyone else, 
      is the basic message that was brought by the Prophet of Allaah ‘Eesa 
      (peace be upon him), as it was the basic message brought by all the 
      Prophets. Allaah says (interpretation of the meaning): 
“And verily, We have sent 
    among every Ummah (community, nation) a Messenger (proclaiming): ‘Worship 
    Allaah (Alone), and avoid (or keep away from) Taghoot (all false deities, 
    i.e. do not worship Taghoot besides Allaah).’ Then of them were some whom 
    Allaah guided and of them were some upon whom the straying was justified. So 
    travel through the land and see what was the end of those who denied (the 
    truth)”

    [al-Nahl 16:36] 

    “And We did not send any Messenger before you (O Muhammad) 
    but We revealed to him (saying): Laa ilaaha illa Ana [none has the right to 
    be worshipped but I (Allaah)], so worship Me (Alone and none else)”

    [al-Anbiya’ 21:25]

    ‘Eesa will bear witness to this call against his people, as 
    Allaah says (interpretation of the meaning):

    “And (remember) when Allaah will 
    say (on the Day of Resurrection): 'O ‘Eesa (Jesus), son of Maryam (Mary)! 
    Did you say unto men: “Worship me and my mother as two gods besides 
    Allaah?”’ He will say: ‘Glory be to You! It was not for me to say what I had 
    no right (to say). Had I said such a thing, You would surely have known it. 
    You know what is in my inner‑self though I do not know what is in Yours; 
    truly, You, only You, are the All‑Knower of all that is hidden (and unseen).

    117. ‘Never did I say to them aught except what You 
    (Allaah) did command me to say: “Worship Allaah, my Lord and your Lord.” And 
    I was a witness over them while I dwelt amongst them, but when You took me 
    up, You were the Watcher over them; and You are a Witness to all things’”

    [al-Maa'idah 5:117]

    With regard to how the followers of this religion deviated 
    after that from pure Tawheed to idolatrous beliefs and the worship of Jesus 
    and his mother besides God, this is something which happened early on in 
    Christian history. We will quote here some evidence to that effect from 
    their own people’s words. Let whoever has ears listen. 

    It says in the American Encyclopedia: 

    The belief in the Oneness of God – as a theological movement 
    – began at a very early stage in history, and in fact it preceded the belief 
    in trinity by many decades. Christianity developed from Judaism, and Judaism 
    firmly believes that there is one God. 

    The path that led from Jerusalem (the home of the first 
    disciples of Christ) to Nicea (where it was decided in 325 CE that Christ 
    was equal to God in essence and eternal nature) can hardly be described as a 
    straight path.

    The doctrine of trinity which was affirmed in the fourth 
    century CE bears no resemblance to the original teachings of Christ 
    concerning the nature of God. Au contraire, it is the opposite, a deviation 
    from that teaching. Hence it developed in opposition to the belief in One 
    God… (27/294). 

    You can refer to the views of some of those Christians who 
    still believe in the Oneness of God in the same American Encyclopedia, 
    27/300-301 

    Will Durant says: 

    When Christianity conquered Rome, the new religion (i.e., 
    Christianity) was infused with the blood of the old idolatrous religion: the 
    title of archbishop, worship for the great mother, and an innumerable number 
    of lords who gave peace of mind and were like who exist in all places and 
    cannot be detected with the senses. All of this 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 assume positions of power. 

    Christianity did not put an end to idolatry, rather it 
    reinforced it. The Greek mind came back to life in a new form, in the 
    doctrines and rituals of the church. The Greek rituals appeared in the 
    rituals of the monastic saints. From Egypt came the idea of the holy 
    trinity, the day of reckoning, eternal reward and punishment, and man’s 
    eternal life in one of the other. From Egypt also came the worship of the 
    mother and child, the mystical union with God, the union which led to 
    Platonism and agnosticism, and the erasing of Christian doctrine. And from 
    Persia came the belief in the return of the Messiah and his ruling the earth 
    for 1000 years. 

    Qissat al-Hadaarah, 11/418 (The 
    Story of Civilization) 

    Despite the element of atheism in the words of Durant, which 
    is something that he is known for, and which is apparent in his claim that 
    the idea of eternal reward or punishment came from the Egyptians, tracing 
    the origins of deviant idolatry in Christianity is no longer a secret, and 
    he is not the only one who has researched them. In his book Christianity 
    and Idolatry, Robertson states that Mithraism, which is a religion of 
    Persian origin, flourished in Persia approximately 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. What concerns us here about this religion is that it 
    says: 

    -That Mithras, after whom it is 
    named, was an intermediary between God and man (for a similar doctrine in 
    Christianity, see Acts 4:12).

    -He was born in a cave or in a 
    corner of the earth (cf. Luke 2:8)

    -His birthday was December 25 
    (which is the day celebrated by the Christians as the day when Jesus was 
    born)

    -He had twelve disciples (cf. 
    Matthew 10:1)

    -He died to save the world (cf. 
    I Corinthians 15:3)

    -He was buried but he came back 
    to life (cf. I Corinthians 15:4)

    -He ascended to heaven in front 
    of his disciples (cf. Acts 1:9)

    -He was called “Saviour” (cf. 
    Titus 2:13)

    -Among his attributes is that he 
    is like a peaceful lamb (cf. John 1:29)

    -The “Divine supper” was held in 
    his memory every year (cf. I Corinthians 11:23-25)

    -One of his symbols was baptism
    

    -Sunday was sacred to them

    The French Orientalist Leon Joteh, in his book “An 
    Introduction to Islamic philosophy” is of the view that the origin of 
    the Christian trinity is to be found in 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.  

    This is what the American writer Draper refers to: 

    Idolatry 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. (p. 337) 

    Although the followers of Christianity gained some power, to 
    the extent that they managed to have Constantine appointed as ruler, they 
    failed to eradicate idolatry completely. As a result of their struggle, 
    their principles became fused with idolatry, from which point there 
    developed a new religion in which Christianity and idolatry were manifested 
    equally. 

    Thus the Christians followed the same path of those 
    disbelievers who had come before them, step by step, as their own Book 
    testifies. And Allaah says (interpretation of the meaning): 
“And the Jews say: ‘Uzayr 
    (Ezra) is the son of Allaah, and the Christians say: Messiah is the son of 
    Allaah. That is their saying with their mouths, resembling the saying of 
    those who disbelieved aforetime. Allaah’s Curse be on them, how they are 
    deluded away from the truth!”

    [al-Tawbah 9:30]

    And Allaah is the Source of strength.