If you believe that Messiah Jesus is a human being what logic you have to guess that he could be a God or son of God? The Messiah Jesus is son of Maryam was a human being. He was born from a woman, he ate and drank, got up and went to sleep, suffered pain and wept… and God (Allaah) is far above all of that, Allah is far than biological characters such as sexuality so how could he be God or son of God?

Note: My Goal is not to argue or to get votes but my goal is to try to understand what is the nature of the logic which works in christianity!
The Christian logic is that God (Allah) incarnated into a baby that was miraculously conceived within Maryam's womb without sexual intercourse, a doctrine called the Virgin Birth.

While being Jesus, God remains the Father simultaneously, so at no point in time God loses any of His divinity nor his greatness.  Obviously it was God's will and His Divine power which made this possible.

Why did He do this?  In order for God (Allah) to save humanity from original sin so all human beings who are willing to make Jesus our Lord and Savior will have eternal life

The logic is:


Only God can fully forgive sin
Only 100% human can be the representative to be in solidarity with humanity


Therefore Jesus has to be fully God and fully Human, a doctrine called the Hypostatic Union.

Addressing the objection:


  By that answer you are decreased the greatness of God (Allah)


In the Christian logic (the doctrine of Trinity), God is still fully God, so there is no reduction of God's greatness by being human.  Jesus as a man, prays to God the Father, who remains fully divine.  It is not like when God incarnated into Jesus there was no God in charge anymore.


  Allah never be similar with human being at all


In the Christian logic, God is the creator of all the universe, including human beings.  Why is it that the Creator cannot assume a human body?  God is Almighty and All powerful.  Islam believes this too.  It is the other way around that is not possible.  No human can become God.  Christians don't believe this is possible.

ADDENDUM


For a much more precise "technical" description of the Incarnation please see the Catholic Catechism on this line from the Nicene Creed: "He was conceived by the power of the Holy Spirit, and born of the virgin Mary."
Sola Gratia's answer is also very good.