How did Christianity become mixed with polytheistic beliefs?

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.