Debate with a Christian: Jesus is the servant of God and His Messenger
Question

Why is it so difficult for the Muslims to believe that Jesus is the only son of God, when it says in the gospel that he is the son of God and he says of God “My Father”?.
Praise be to Allah.We have previously explained that the Gospel in which we believe, and no one's Islam is valid unless he believes in it, is not the gospels that are in the hands of the Christians nowadays. Rather the Gospel in which we believe is that which was brought by ‘Eesa (Jesus – peace be upon him) from Allah. As for that which is in the hands of the Christians today, it is something else, and they themselves do not claim that Jesus is the one who brought it or wrote it. See question no. 47516. 
As that is the case, what the Christians claim about the Gospels stating that Jesus is the son of God and that God is his father –exalted be Allah above having a son or a wife – does not count as any kind of proof against us, because we believe that that is something that was fabricated by human beings, and it is not part of the religion of Jesus (peace be upon him) or the religion of any other Messenger. 
We believe that the Gospels that are in people’s hands today, in which the Christians believe, have been tampered with and changed, and are still being tampered with from time to time, so that there is nothing left in the form in which the Gospel was revealed from Allah. Here we would point out that the Gospel which speaks most of the belief in the trinity and the divinity of the Messiah (peace be upon him), so that it has become a reference-point for the Christians in their arguments in support of this falsehood, is the Gospel of John. This Gospel is subject to doubts about its authorship even among some Christian scholars themselves, as is not the case with the other Gospels in which they believe. This is an ancient doubt which goes back to the second century CE according to their own history. 
Professor Stadlin says:  The entire Gospel of John was written by one of the students of the Alexandrian school. One sect, in the second century, rejected this Gospel and everything that was attributed to John. 
In the Encyclopaedia Britannica it says:  
As for the gospel of John, it is undoubtedly fabricated. Its author wanted to pitch two of the disciples against one another, namely St. John and St. Matthew. 
This writer who appears in the text claimed that he was the disciple who was loved by the Messiah, and the Church took this at face value and affirmed that the writer was the disciple John, and it put his name on the book, even though the author was not John for certain. This book is like the books of the Torah, in that there is no connection between them and the one to whom they are attributed. We feel sorry for those who did their utmost to make the connection, between this philosopher who wrote the book in the second century, and the disciple John the fisherman, for their efforts were to no avail and with no guidance.  
Quoted from Muhaaraat fi’l-Nasraaniyyah by Shaykh Muhammad Abu Zahrah. 
It is strange indeed that they cast aspersions on the authorship of this Gospel which they affirm was written especially to support this falsehood, the false belief in the divinity of the Messiah, which is ignored in the other gospels, until this gospel was written, at the least. Yoosuf al-Khoori says: John wrote his Gospel at the end of his life, at the request of the bishops of Asia and elsewhere. The reason for that is that there were sects that denied the divinity of the Messiah, so they asked him to prove it, and to highlight that which Matthew, Mark and Luke had neglected in their Gospels. 
(op.cit., p. 64) 
Regardless of the doubts about the authorship of the Gospels in general, and of the Gospel of John in particular, the phrases that they quote from these Gospels do not support the point they are trying to make, rather it is a spider’s web to which they are clinging, as Allah says of them and others like them (interpretation of the meaning): 
“The likeness of those who take (false deities as) Awliya’ (protectors, helpers) other than Allah is the likeness of a spider who builds (for itself) a house; but verily, the frailest (weakest) of houses is the spider’s house if they but knew” [Al-‘Ankaboot 29:41]
The Bible in which it says that the Messiah is the son of God is the same Bible in which the lineage of the Messiah ends with Adam (peace be upon him), and he too is described as a son of God.  
“Now Jesus himself was about thirty years old when he began his ministry. He was the son, so it was thought, of Joseph, the son of Heli … the son of Seth, the son of Adam, the son of God” [Luke 3:23-38] 
This is the same Bible that describes Israel in the same terms: 
“Then say to Pharaoh, 'This is what the LORD says: Israel is my firstborn son” [Exodus 4:22] 
Something similar appears in the Book of Hosea: 
“When Israel was a child, I loved him, and out of Egypt I called my son” [Hosea 11:1] 
The same is said of Soloman (peace be upon him): 
“He said to me: 'Solomon your son is the one who will build my house and my courts, for I have chosen him to be my son, and I will be his father” [I Chronicles 28:6] 
Were Adam, Israel and Soloman all other sons of God, before the Messiah (peace be upon him)? Exalted be Allah far above what they say! 
Indeed, in the Gospel of John itself there is an explanation of what is meant by this being a son; it includes all the righteous servants of God, so there is nothing unique about Jesus or any other Prophet in this regard. 
“But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name” [John 1:3] 
Something similar appears in the Gospel of Matthew: 
“Blessed are the peacemakers, for they will be called sons of God” [Matthew 5:8-9] 
This usage of the word “son” in the language of the Bible is a metaphor for the righteous servant of God, without it implying anything special or unique about the way in which he is created, or describing him literally as the offspring of God. Hence John says: 
“How great is the love the Father has lavished on us, that we should be called children of God!” [1 John 3:1] 
For the same reason Adam is also called a son of God – exalted be Allah far above that. 
There remains the issue of Eesa (peace be upon him) being described as a son of God, and what they fabricated about the Lord of the Worlds, saying that He was the father of the Messiah (peace be upon him). This too is not unique in the language of the Gospels: 
“Jesus said, Do not hold on to me, for I have not yet returned to the Father. Go instead to my brothers and tell them, 'I am returning to my Father and your Father, to my God and your God.'” [John 20:17] 
In one text he says that God is a father to them too, and that God is the God of them all. 
So let them say if they wish that we are all the children and the beloved of God, as God said of their forefathers. In that case there is nothing special about the Messiah so that they should worship him instead of Allah. Or let them be stubborn and follow something other than true guidance, with no clear Book. This is something that anyone could do. 
, the Lord of the heavens and the Lord of the earth, the Lord of the Worlds, for the blessing of Islam that He has bestowed upon us. 
O Allah, guide us to Your straight path, The way of those on whom You have bestowed Your Grace, not (the way) of those who earned Your Anger, nor of those who went astray.