We find that the Qur’an presents God being One, Ahad, or as unitarian in the sense that it rejects the personal relationships within the Godhead (Sura 112:1-4 and others). On the other hand we find that Christian monotheism or the belief in one true God in the light of many references in the Bible about God teaches that God is the Father, the Son and the Holy Spirit. From this Christians understand that although he is One True God, he is trinitarian.
Muslims indeed object that the word Trinity does not appear in the Bible for the understanding of the monotheism that Christians present. Well, the same goes for the Islamic monotheism the word Tawhid as adopted in the Islamic law system indeed does not appear in the Qur’an either. For a Muslim, God is aloof and that He created us so that our relationship with Him will be always and only as of a Master and servant (Sura 19:33).
In opposition to that, the Christian understanding is that before creating the universe, God was already in a relational life. Within the Godhead, the Father related to the Son, the Son to the Father, the Father and Son to the Holy Spirit, and so on. God created us, our family and our community structure after his trinitarin image. Our relationship thus with Him goes beyond as of a Master and servant. In obedience to him our relationship with him is of a Father and son through His Holy Spirit. Now, it does not mean that Jesus is just a human being and son of God as we are. Jesus is the unique Son of God. How? The Christian Scriptures mentions him as leaving his divine status and becoming like us after the imprint of first Adam so to be the second Adam on this earth. And as through the first Adam we have inherited our physical being along with a fallen nature after the fall, now through faith by our belief in Christ as the second Adam we are being restored to the spiritual, the eternal life with God and in his realms again.
The Christian Scripture is quite clear about Jesus being in his eternal essence with God before even the foundation of the universe. As God is, Jesus as being His word or Son with the Holy Spirit in union was there. How? This has not been very much explained in the Scriptures. Why? Well if everything had been explained through the standard of reasoning we have, first of all it will still be impossible and we will lack the comprehension in the fallen state we are. However, the Day is coming when we will know Him as He is and will continue t know Him more and more. At present He has revealed as much as is enough for us.
You may want to read also another article, 'Who is God?' click here.