Indeed Islam is the religion of the truth, the perfected religion, and the religion of pure monotheism. Islam's entire foundation and basis revolves around the pure monotheism of God, and it is through understanding this pure monotheism of God that one opens up a relationship with his Creator, and how one will attain the fruits of salvation.
So let us compare the truthful monotheism of Islam with the Trinitarian Polytheism of Christianity, and indeed polytheism is the opposite of monotheism, it is the doctrine that dooms somebody, and it is the doctrine that leaves humans to become lost and confused, therefore it is essential for monotheists as myself to reach out for this polytheism and crush it with the truth.

Islam
In the mighty religion of Islam we believe in Tawheed of Lordship, that is to say that Allah created everything, and is in control of everything, and that there is no other almighty God existent with him, as the Quran mentions:
And your Allah is One Allah: There is no god but He, Most Gracious, Most Merciful. (2:163)
Knowest thou not that to Allah (alone) belongeth the dominion of the heavens and the earth? He punisheth whom He pleaseth, and He forgiveth whom He pleaseth: and Allah hath power over all things. (5:40)
That is Allah, your Lord! there is no god but He, the Creator of all things (16:102)
To Allah doth belong the dominion of the heavens and the earth, and all that is therein, and it is He Who hath power over all things. (5:120)
So there is only one true God, who is the creator of everything, and who is the one who is in control of everything, everything is under him and his power, there is no other God but him Alone, and this is pure monotheism.


Trinity
According to the Trinity you have the Father, Son, and the Holy Spirit, all three of these persons are fully equal to the other, and each three of these persons are fully God, meaning all three of them are the creators of everything, which also means that all three of them are in control of everything as well. This obviously contradicts the obvious message of monotheism for there is only ONE true creator, and there is only ONE true Master who is in control of everything, if you have three gods who are doing all of these things then you have three almighty's and three all powerfuls! Yet there is only one All Mighty, and one All Powerful, who has no rivals or anyone or any ?person' who can rival or do what he does, if he has others who can rival and do what he does, and if he has others who also share in his complete dominion and control then he is not unique and this obviously contradicts with the pure monotheism of Him being the only true creator who is in control of all affairs.
Hence Trinitarians believe in Three Creators, in Three All Might's, and this contradicts God's monotheism of Lordship.


Islam
In the mighty religion of Islam we believe in Taweed of Worship, that is to say that all Worship belongs to God alone, and to none other than him, you only worship Him alone, and no one else, as the Quran mentions:
And I have not created the jinn and the men except that they should Worship Me. (51:56)
"Verily, I am Allah: There is no god but I: So Worship thou Me (only), and establish regular prayer for celebrating My praise. (20:14)
So we the Muslims only pray to Allah alone, and to none others, this is pure monotheism and the complete opposite of Polytheism.


Trinity
The Trinitarians worship three persons, the Father, Son, and the Holy Spirit; hence their worship is tainted and not pure as it is directed towards three persons! Worship should be given to the one and true God alone, Trinitarians will reply back by saying yes they worship God alone, but worship the three ?persons', these are semantical games of nonsense, whether the Christian says person, or being, it doesn't matter. Allah is indeed one being, yet one can also say that Allah is one person, hence when Trinitarians say ?person' it makes no difference at all, we Muslims worship the one true God Allah, who is ONE, whether you want to say Being, or Person, he is One in the absolute sense and there is no two ways about it. Worship is something very sacred and unique, this is why it should be solely down to one being, one person, and not shared for three persons, but for the One true person alone.
Furthermore Trinitarians believe that each ?person' is FULLY God, not that each ?person' makes up a part of God, so if we have three ?persons' who are fully God, and these three ?persons' are distinct from one another then we are left with three Gods. So the Trinitarians worship three Gods according to their own doctrine! Hence this is clear polytheism and major shirk of the worst kind, and this is the doctrine that all prophets and messengers came to oppose and fight against!

Islam
In the mighty religion of Islam we believe in Tawheed Al-Asmaat wa Sifaat, that is to say that Allah has unique and exclusive Names and Attributes for him alone. So for instance one of Allah's Names is Al-Aziz which is the All Mighty, hence this Name and Attribute of what this Name constitutes is uniquely for Allah alone, and a creation cannot be given a Name and Attribute of God for there is nothing like God:
(He is) the Creator of the heavens and the earth: He has made for you pairs from among yourselves, and pairs among cattle: by this means does He multiply you: there is nothing whatever like unto Him, and He is the One that hears and sees. (42:11)
Hence we Muslims believe that Allah is completely unique and that there is nothing like him, and this is pure monotheism, for a pure monotheist cannot compare anything to the All Mighty God


Trinity
Trinitarians openly give unique names and attributes of God to the created being Jesus, hence they have committed polytheism by ascribing unique and incomparable attributes of God to that of the creation.
Furthermore since Christians believe in three ?persons' who are all God, this then means that all of these three Gods have all of these attributes which means they are no longer unique as another God (?Person') already has them! Hence you have THREE All Might's, THREE all knowing, and THREE all Glorious ones!
Thirdly, Christians believe that Jesus was a human being, he had flesh and bones and he bled blood, and he also ate and cried and slept! This obviously contradicts the pure monotheism of there is nothing like God, even Paul himself condemned people for lowering God onto the creation:
Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Professing themselves to be wise, they became fools, And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things. Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever (Romans 1:21-25)
It's bad enough to give other persons unique Names and Attributes of God to other persons, yet it becomes even worst when you insist that God himself is a man with flesh and bone, essentially he is the creation as well!
So in this respect Trinitarians completely go off the mark in terms of God's unique Names and Attributes.

Conclusion
In conclusion I would like to ask the Trinitarians to worship the one true Person that is God, to believe that the one true Person that is God is the one who is the creator and master of all things, and to believe that the one true Person that is God is unique in his Names and Attributes and is not comparable to anything!
And Allah Knows Best!
http://muslim-responses.com/Monotheism_Vs_Polytheism/Monotheism_Vs_Polytheism_