Christians honestly see that they believe in One God, but in three 'Persons'. Their basic concept or understanding of God can be reflected in the Biblical passage, "As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.” " Matthew 3:16-17 Muslims do not see these verses or the Nicene Creed as a statements of Divine unity.
The Nicene Creed starts with, "We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible." Muslims would fully agree that Christians believe in One God if they stopped there; however, this statement of unity is immediately negated with, "And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds (æons), Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost of the Virgin Mary, and was made man; he was crucified for us under Pontius Pilate, and suffered, and was buried, and the third day he rose again, according to the Scriptures, and ascended into heaven, and sits on the right hand of the Father from thence he shall come again, with glory, to judge the quick and the dead; whose kingdom shall have no end." There are 20 words of the Nicene Creed allocated to the Father and 128 to Jesus which clearly reflects the Christian focus on Jesus in their worship. There is no Muslim who can disagree with the essence of the first statement except for the use of the word 'father' which implies having offspring. Likewise, there is no Muslim who will accept the second statement as they see it as ascribing partners (a human being, Jesus) with the One God (Allah, glorified and exalted is He).
Perhaps the single most quoted verse in the entirety of the Bible is John 3:16, "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life." The word 'begotten' is defined as "(of offspring) generated by procreation; to procreate as the father". The word procreate is defined as "to beget and conceive (offspring), to produce or create; originate, to beget or bring forth (offspring)". Since we know that the Father did not physically sire or beget Jesus (astaghfir Allah), what does 'begotten' mean? A simplistic meaning for 'begotten' implies a beginning for a descendant (son or daughter) that began through the actions of a male parent (father). The word 'conceive' is the passive flip-side of this coin that is applied to the mother as indicated by the definition, "to become pregnant with (offspring)". Since we know that the Father did not physically beget Jesus in the normal human means, then the word begotten can be understood figuratively in the sense of "to produce or create; originate" for the term "begotten" which further sheds light on the title "SON OF GOD" as metaphorical, symbolic, or figurative. Can not this same figurative meaning be applied to Adam, which the Qur'an references with, "Verily, the likeness of Jesus before Allah is the likeness of Adam? He created him from dust, then (He) said to him: "Be!" - and he was." 3:59?
My understanding is that Allah's (swt) existence is unlike our existence. Our existence is defined by the miniscule space occupied by our bodies and by the infinitesimally small amount of time between our conception and our death. My understanding is that Allah (swt) is not bound by our human limitations of the space-time continuum. We do not perceive Allah (swt) as occupying one portion of space over here while not occupying another portion over there, but neither do we say that Allah (swt) physically exists everywhere and in everything at the same time (astaghfir Allah). The same can be said for any point in time from before the moment of creation until after its destruction.
Allah's (swt) existence is the only true reality which brings us to the question of what does it mean to be a man? What is the nature or central essence of our existence? We have a physical body that contains a few trillion cells each with 46 chromosomes that are all the same, but are unique to each individual (except identical twins). Am I really only that Caucasian male body, or am I more than my body? We go through stages of development and decline, but are we defined by any single moment between our birth and death? Do we cease to exist with our death? I have a certain ability to think and understand through the activity of my brain, but does that define who I am along the lines of, "I think, therefore I am"? When my brain ceases to function, do I then cease to exist? I have a certain will to act and to move my body a certain way or think about the solution to some problem. Is that capability or will to act and think define who I am, or is it the choices I make along the way? I have certain beliefs about the unseen, but are those beliefs what defines who I am? I could go on and on with asking the question of "Who am I?" with no clear answer other than an accumulation of choices, actions and intentions made be some essence (the soul) that exists within my body between my conception and death. If, in a sense, my essential existence is a mystery to me, then how much more so the existence of Allah who is beyond the limitation of occupying a human body (astaghfir Allah)?
If there is an intangible aspect of my being (soul) that defines who I am, then was that essential 'human' essence lacking in Jesus and replaced with a 'God' essence, or did the human essence coexist with the God essence inside Jesus' body? If the human essence coexisted with the God essence, then which one was dominant and took precedence? Matthew 26:38-39 should shed light on this, "Then He said to them, “My soul is exceedingly sorrowful, even to death. Stay here and watch with me.” He went a little farther and fell on His face, and prayed, saying, “O My Father, if it is possible, let this cup pass from me; nevertheless, not as I will, but as You will.” This shows to me that Jesus had a human soul and that his central essence was different and distinct from that of the Father; therefore, Jesus was fully man and in no way, shape or form was he 'God Incarnate.'
Muslims believe that Jesus was born miraculously to the virgin, Mary, without any human father; however, we reject the notion that Jesus was the 'only begotten Son of God' and at the same time literally God Incarnate. We do not believe that it behooves the majesty and glory of God for Him to become embodied inside one of His creatures or for Him to become fragmented into three persons with one coming up from being baptized, another descending in the form of a dove and a third speaking from Heaven of the first as His son. We believe that Jesus was a blessed Prophet, Messenger and Servant of Allah in the same manner that Moses, David, and Muhammad were (peace be upon them). May Allah (swt) remove the confusion from our minds so that we worship Him alone without ascribing any partners with Him. I write from my own limited understanding and I ask God to forgive me for any errors I may have made.