Discussion With a Christian: The Oneness of God is the Message of Jesus and all the Prophets, Peace be Upon Them
Question

Why do you shun the truth and say that Jesus is only a Messenger instead of regarding him as the son of God? Jesus is the only son of the Lord and that is the truth!!
Praise be to Allah.Thank you for giving us the opportunity to communicate and discuss the truth which we all need to know and follow, no matter what our previous differences of opinion and beliefs. It is only by means of this, I mean by knowing the truth and following it, that we can free ourselves of our sins, as it says in your Bible: 
“Then you will know the truth and the truth will set you free”
John 8:31 – New International Version (NIV) 
So let us pause for a while and think about this truth, as it is stated in your Bible despite all the distortions and changes that have happened to it!
The message that was brought by the Messiah (peace be upon him) was the call to worship God, the One, the Lord of the Messiah and the Lord of the worlds: 
“Now this is eternal life: that they may know you, the only true God, and Jesus Christ, whom you have sent”
John 17:3 – NIV 
“A certain ruler asked him: ‘Good teacher, what must I do to inherit eternal life?’
‘Why do you call me good?’ Jesus answered. ‘No one is good – except God alone.’”
Luke 18:18-19 – NIV 
“The devil led him up to a high place and showed him in an instant all the kingdoms of the world.
And he said to him, ‘I will give you all their authority and splendour, for it has been given to me, and I can give it to anyone I want to.
So if you worship me, it will all be yours.’
Jesus answered, ‘It is written: “Worship the Lord your God and serve him only.”’”
Luke 4:5-8 – NIV 
Believing in and worshipping Allah alone, besides Whom there is no other god, is the greatest teaching brought by the Messiah, and it is the greatest teaching brought by all the Prophets. 
“One of the teachers of the law came and noticed them debating. Noticing that Jesus had given them a good answer, he asked him, ‘Of all the commandments, which is the most important?’
‘The most important one,’ answered Jesus, ‘is this: “Hear, O Israel, the Lord our God, the Lord is one.
Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.”
The second is this: “Love your neighbour as yourself.” There is no commandment greater than these.’
‘Well said, teacher,’ the man replied. ‘You are right in saying that God is one and there is no other but him.
To love him with all your heart, with all your understanding and with all your strength, and to love your neighbour as yourself is more important than all burnt offerings and sacrifices.’
When Jesus saw that he has answered wisely, he said to him, ‘You are not far from the kingdom of God.’ And from then on no one dared ask him any more questions.”
Mark 12:28-34 – NIV 
But do not think that this advice was given to Israel or to his own people only. Rather this is the basis of the teachings of all the Prophets. The same advice appears in the Gospel of Matthew, in similar wording, after which he says: 
“All the Law and the Prophets hang on these two commandments.”
Matthew 22:39 – NIV  
This belief in the oneness of God is indeed the message of all of the Prophets. Allah says (interpretation of the meaning): 
“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 (Allah)], so worship Me (Alone and none else)”
[al-Anbiya’ 21:25]
This is the basic message to which the Messiah called people and warned them against differing from that. Allah says (interpretation of the meaning): 
“Surely, they have disbelieved who say: ‘Allah is the Messiah [‘Eesa (Jesus)], son of Maryam (Mary).’ But the Messiah [‘Eesa (Jesus)] said: ‘O Children of Israel! Worship Allah, my Lord and your Lord.’ Verily, whosoever sets up partners (in worship) with Allah, then Allah has forbidden Paradise to him, and the Fire will be his abode. And for the Zaalimoon (polytheists and wrongdoers) there are no helpers”
[al-Maa'idah 5:72]
This is the basic principle on which we should all agree. Allah says (interpretation of the meaning): 
“Say (O Muhammad): ‘O people of the Scripture (Jews and Christians): Come to a word that is just between us and you, that we worship none but Allah (Alone), and that we associate no partners with Him, and that none of us shall take others as lords besides Allah.’ Then, if they turn away, say: ‘Bear witness that we are Muslims’”
[Aal ‘Imraan 3:64] 
It is alien to true Christianity, this futile attempt to reconcile the belief in the Oneness of God, which is the message brought by the Prophets, and which is clearly stated in their Bible, and affirmed in the Torah in particular, with their belief in the trinity. 
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 
It is difficult to comprehend, so no wonder you were never able to comprehend it. But what is strange is that you believe in something that is impossible to understand, unless we deceive ourselves and say that this understanding will come on the Last Day: 
“We understand that as much as our minds are able to, and we hope that we will understand it more clearly in the future, when the veil is removed from all things in heaven and on earth. But for now, the extent to which we do understand it is enough!” 
Yes, the truth will become perfectly clear to you in the future, as it is clear to us today, praise be to Allah, on the Day on which Allah will gather the Messengers and make them bear witness concerning their nations. Allah says (interpretation of the meaning): 
“And (remember) when Allah 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 Allah?’  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).
‘Never did I say to them aught except what You (Allah) did command me to say: Worship Allah, 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. (This is a great admonition and warning to the Christians of the whole world).
‘If You punish them, they are Your slaves, and if You forgive them, verily, You, only You, are the All‑Mighty, the All‑Wise.’
Allah will say: This is a Day on which the truthful will profit from their truth: theirs are Gardens under which rivers flow (in Paradise) — they shall abide therein forever. Allah is pleased with them and they with Him. That is the great success (Paradise).
120. To Allah belongs the dominion of the heavens and the earth and all that is therein, and He is Able to do all things”
[al-Maa’idah 5:116-120].