Claims of Christ Questioned
The Christian Bible is composed of 66 books written over a period of several hundred years with at least 40 men copying down what God had said; and in all aspects the Bible predates the Koran; which was given by Muhammad over a very short period, nearly 600 years after Christianity was founded.
You are right, although, the word Trinity is not mentioned in the Bible; however, the concept is certainly there.
So, I will not defend the word "Trinity", but I will defend the concept.
To begin with Muslims believe that Christians worship three Gods; however, Christians do not worship three gods. They worship only one God (Allah) who is Father, Son, and Holy Spirit. These three are one, so that is why we use the term Trinity to describe who God is.
For example, traditionally Muslims attribute 99 names to Allah to describe Him; yet no one that I know accuses a Muslim of worshiping 99 gods.
More specifically, however, Christian scholars, just like Muslim scholars distinguish between the attributes of God and His Divine essence. Christians also ascribe many attributes to God. However, when we Christians refer to The Trinity we do not talking about His attributes—that is, His names or what He is like; but rather Who He is—His Divine essence.
A man, for instance, is a body, a mind and a soul; but man is not three men, he is one. Similarly, God is the Father, the Son, and the Holy Spirit. In other words, that is His essence, just as the essence of man is what man is.
Therefore, we must carefully distinguish between what God is like, from who He is.
Christians believe that He is in essence One, just as Muslims do; and that essence is expressed in and through who He is. Stop and think for a moment. Since God is self-sufficient and never had to depend on anyone or anything for his total existence, does it not seem reasonable to believe that He had the capacity to communicate before creation?
Now, if this is true, then with whom did He communicate prior to the creation? He must have communicated with someone other than His creation. The Bible (the Holy Injil) and the Qur’an clearly tells us that God is the Creator and that absolutely nothing was created without Him. How then is God complete unless He is fully capable of communicating both within Himself prior to creation and to us His creation in the expanse of time?
The answer is, of course, that He is so great and His essence is so unfathomable that we will never be able to fully comprehend or exhaustively describe Him; however, we must be careful not to limit Him, either. Therefore, we must depend on Him to tell us Who He is, even when at first we may not comprehend who He says He is. In the Bible, He tells us who He is. He tells us that He is the Holy Spirit; He is the Son, Jesus; and He is the Father. That is these three are the nature of His (Allah) essence.
Do I understand it? No! But, I do believe it because the Bible declares it.
For easy reference, here are some of the verses that declare who God is:
Jesus said, ‘He who has seen me has seen the Father’ (John 14:9), and ‘I am the way and the truth and the live. No one comes to the Father except through me’ (John 14:6)
Jesus also said that “God is a Spirit’ (John 4:24).
So, we see that God has an essence of three personas—or as some Christian theologians say, persons. However, when we use that word, we do not mean three separate individuals; but three personal relationships. This is simply a way of describing the nature of God’s essence. He is the Father, Son, and Holy Spirit and we relate to Him as such.
The Qur’an also recognizes The virgin birth of Jesus was a sign from God. The Qur’an says, "And (remember) her who guarded her chastity: We breathed into her of Our spirit, and We made her and her son a sign for all peoples." (Qur'an 21:91)
To whom was God referring to when he said, “Our spirit” in the sura above? Christians believe that the Father, The Son (Isa) and The Holy Spirit are the Divine essence of God and, therefore, present at creation and could be the only “Our spirit” worthy enough to form a union with God and man.
Could it be, therefore, that God and Jesus in this instance is the Divine “our” which sent forth their spirit? Why would any creature’s conception be described in this manner unless they were indeed Divine?
I am from my father, as was Jesus. I was not created; neither was Jesus. Adam was created from dust, I was not; neither was Jesus. Jesus, came from and was God (Allah).
John, one of the early disciples of Jesus put it this way in the Holy Bible, “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God.” (John 1:1-2)
And, who is this Word? Both the Bible and the Qur’an say that this is Jesus (Isa). So, since this declaration is a fact agreed upon in both the Bible and in the Qur’an, how do we then reconcile this with the contradictions we find in both texts?
Admittedly, we cannot reconcile all of these contradictions, but in this instance, reason alone argues that you cannot have the “our spirit” entering Mary to conceive a child—whom both the Bible and the Qur’an call “the word of God”—without that Word (Isa) being in the form of God.
This fact of reason, however, we must accept by faith. So, although, we may not be able to describe God completely we do know that we can relate to Him personally since the Word became flesh and lived among us. Furthermore, many of us have also experience God’s great mercy in the forgiveness of sin through Jesus Christ, the Son or Word of God.
Otherwise, how can we say with any confidence that God is The Most Merciful in Essence, The Compassionate, the Ever Forgiving, or The Loving and the Kind One unless we have some visible proof that He is. Jesus is that proof! He came as God in the flesh; and, although, he never sinned he was willing and did become our Savior by dying on the Cross suffering our punishment for us and was affirmed as such by God the Father when He raised Jesus the Son from the grave by the power of His Spirit.
 Suras 3:6, 4:158, 9:40, 48:7, 59:23
 Suras 6:102, 13:16, 39:62, 40:62, 59:24
 It is interesting to note that the Qur’an seems to recognize Jesus as the Christ, which most definitely is to associate Him with Divinity.
Here is the sura translated by three outstanding Arabic scholars: 003.045
YUSUFALI: Behold! the angels said: "O Mary! Allah giveth thee glad tidings of a Word from Him: his name will be Christ Jesus, the son of Mary, held in honor in this world and the Hereafter and of (the company of) those nearest to Allah;
PICKTHAL: (And remember) when the angels said: O Mary! Lo! Allah giveth thee glad tidings of a word from him, whose name is the Messiah, Jesus, son of Mary, illustrious in the world and the Hereafter, and one of those brought near (unto Allah).
SHAKIR: When the angels said: O Marium, surely Allah gives you good news with a Word from Him (of one) whose name is the '. Messiah, Isa son of Marium, worthy of regard in this world and the hereafter and of those who are made near (to Allah).
 Ingil John 1: 1-32; Qur'an 4:171
 Ar-Rahman The All Beneficent, The Most Merciful in Essence, The Compassionate, The Most Gracious Beginning of every chapter except one, and in numerous other places
2 Ar-Rahim The Most Merciful, The Most Merciful in Actions Beginning of every chapter except one, and in numerous other places
Al-Ghaffar The Ever Forgiving 20:82, 38:66, 39:5, 40:42, 71:10
Al-Wadud The Loving, The Kind One 11:90, 85:14
 In the Qur’an only God is Al-Ba'ith The Raiser of The Dead 22:7; and Al-Muhyi The Giver of Life 7:158, 15:23, 30:50, 57:2; and since He is also Al-Qadir The All Able 6:65, 36:81, 46:33, 75:40
Al-Aziz The Almighty, The Self Sufficient, The Most Honorable 3:6, 4:158, 9:40, 48:7, 59:23, he is fully capable of providing and giving this great gift of forgiveness to us through Jesus Christ.