May peace and blessings be upon you all.
Thank you sister Hana for answering the questions of our Christian brothers. Now it's interesting that Paul was mentioned and I taught we should first study how the doctrine of the Trinity got started. Since the topic is about Jesus' divinity, it's interesting to know that this concept was formulated a few hundred years after his [PBUH] death. Here is a brief history of events leading to the establishment of the doctrine:
Who invented the trinity?
           The three monotheistic religions -  Judaism,  Christianity,  and Islam -
    all purport to  share  one  fundamental  concept:  belief  in God as the
    Supreme Being, the  Creator  and  Sustainer  of  the  Universe. Known as
    "tawhid" in Islam, this concept of  the  Oneness  of God was stressed by
    Moses in a Biblical passage known as the "Shema," or the Jewish creed of
    faith: "Hear, O Israel: The Lord our God is one Lord." (Deuteronomy 6:4) 
    It was repeated word-for-word  approximately  1500  years later by Jesus
    when he said: "...The first of all the  commandments is, Hear, O Israel;
    the Lord our God is one Lord." (Mark 12:29) 
    Muhammad came along  approximately  600  years  later, bringing the same
    message again: "And your God is  One  God:  There is no God but He, ..."
    (The Qur'an 2:163) 
    Christianity has  digressed  from  the  concept  of  the Oneness of God,
    however, into a vague and mysterious doctrine that was formulated during
    the fourth century. This  doctrine,  which  continues  to be a source of
    controversy both within and without  the Christian religion, is known as
    the Doctrine of the Trinity.  Simply  put, the Christian doctrine of the
    Trinity states that God  is  the  union  of  three  divine persons - the
    Father, the Son and the Holy Spirit - in one divine being. 
    If that concept, put  in  basic  terms,  sounds  confusing,  the flowery
    language in the actual text of the  doctrine  lends even more mystery to
    the matter: 
    "...we worship one God in Trinity, and  Trinity in Unity... for there is
    one Person of the Father, another of  the Son, another of the Holy Ghost
    is all one... they are not three  gods,  but  one God... the whole three
    persons are co-eternal and  co-equal...  he  therefore that will be save
    must thus think of the Trinity..." (excerpts from the Athanasian Creed) 
    Let's put this together in a different form: one person, God the Father
    +one person, God the Son + one person, God the Holy Ghost =one person,
    God.  
    It is said that Athanasius,  the  bishop  who  formulated this doctrine,
    confessed that the more he wrote on  the matter, the less capable he was
    of clearly expressing his thoughts regarding it. 
    How did such a confusing doctrine get its start? 
2. Trinity in the Bible
    
    References in the Bible to  a  Trinity  of  divine  beings are vague, at
    best. 
    In Matthew 28:19, we find  Jesus  telling  his  disciples  to go out and
    preach to all nations. While the "Great Commission" does make mention of
    the three persons who later become components of the Trinity, the phrase
    "...baptizing them in the name of the Father, and of the Son, and of the
    Holy Ghost" is quite clearly an addition to Biblical text - that is, not
    the actual words of Jesus - as can be seen by two factors: 
    1) Baptism in the early Church, as discussed by Paul in his letters, was
    done only in the name of Jesus; and 
    2) The "Great Commission" was found in the first gospel written, that of
    Mark, bears no mention  of  Father,  Son  and/or  Holy  Ghost - see Mark
    16:15. 
    The only other reference in the  Bible  to a Trinity can be found in the
    Epistle of I  John  5:7,  Biblical  scholars  of  today,  however,  have
    admitted that the phrase "...there are three that bear record in heaven,
    the Father, the Word, and the Holy  Ghost:  and  these three are one" is
    definitely a "later addition" to  Biblical  test, and it is not found in
    any of today's versions of the Bible. 
    It can, therefore, be  seen  that  the  concept  of  a Trinity of divine
    beings was not an idea put forth  by  Jesus or any other prophet of God.
    This doctrine, now subscribed to  by  Christians  all over the world, is
    entirely man-made in origin. 
3. The Doctrine Takes Shape 
       While Paul of Tarsus, the man  who  could  rightfully  be considered the
    true founder of Christianity, did formulate  many of its doctrines, that
    of the Trinity was not among them.  He  did, however, lay the groundwork
    for such when he put forth the idea of Jesus being a "divine Son." After
    all, a Son does need a  Father,  and  what  about  a  vehicle  for God's
    revelations to man? In essence, Paul named the principal players, but it
    was the later Church people who put the matter together. 
       Tertullian, a lawyer  and  presbyter  of  the  third  century  Church in
    Carthage, was the first to use the word  "Trinity" when he put forth the
    theory that the Son and the Spirit participate  in the being of God, but
    all are of one being of substance with the Father.
4. Formal Doctrine is Drawnup
    When controversy over the matter  of  the Trinity blew up in 318 between
    two church men from Alexandria -  Arius,  the deacon, and Alexander, his
    bishop - Emperor Constantine stepped into the fray. 
    Although Christian dogma was a  complete  mystery to him, he did realize
    that  a  unified  church  was  necessary  for  a  strong  kingdom.  When
    negotiation failed to settle  the  dispute,  Constantine  called for the
    first ecumenical council in Church history in order to settle the matter
    once and for all. 
    Six weeks after the 300  bishopsfirst  gathered  at  Nicea in 325, the
    doctrine of the Trinity was hammered  out. The God of the Christians was
    now seen as having  three  essences,  or  natures,  in  the  form of the
    Father, the Son, and the Holy Spirit. 
5.Church Puts its Foot down
      The matter was far from settled, however, despite high hopes for such on
    the part of Constantine. Arius and  the new bishop of Alexandria, a man
    named Athanasius, began arguing over the matter even as the Nicene Creed
    was being signed; "Arianism"  became  a catch-word from that time onward
    for anyone who did not hold to the doctrine of the Trinity. 
    It wasn't until 451, at the Council of Chalcedon that, with the approval
    of the Pope, the  Nicene/Constantinople  Creed was set as authoritative.
    Debate on the matter was no longer  tolerated;  to speak out against the
    Trinity was now considered  blasphemy,  and  such earned stiff sentences
    that  ranged  from  mutilation  to   death.  Christians  now  turned  on
    Christians, maiming and  slaughtering  thousands because of a difference
    of opinion.
6. The Debate Continues...
    Brutal punishments and even death did  not stop the controversy over the
    doctrine of the Trinity,  however,  and  the  said controversy continues
    even today. 
    The majority of  Christians,  when  asked  to  explain  this fundamental
    doctrine of their faith,  can  offer  nothing  more  than  "I believe it
    because I was told to do so." It  is  explained  away as "mystery" - yet
    the Bible says in I Corinthians 14:33 that "... God is not the author of
    confusion..." 
    The Unitarian denomination of Christianity  has kept alive the teachings
    of Arius in saying that God is one;  they do not believe in the Trinity.
    As a result, mainstream Christians abhor  them, and the National Council
    of Churches has refused their  admittance.  In Unitarianism, the hope is
    kept alive that Christians  will  someday  return  to  the preachings of
    Jesus: "...Thou shalt worship the Lord  thy God, and Him only shalt thou
    serve." (Luke 4:8)
7. Islam and the Matter of The Trinity
    While Christianity may have a problem  defining the essence of God, such
    is not the case in Islam. 
    "They do blaspheme who say:  Allah  is  one  of  three in a Trinity, for
    there is no god except One God."  (Qur'an  5:73) It is worth noting that
    the Arabic language Bible uses the name "Allah" as the name of God. 
    Suzanne Haneef, in her book WHAT  EVERYONE  SHOULD  KNOW ABOUT ISLAM AND
    MUSLIMS (Library of Islam, 1985), puts  the matter quite succinctly when
    she says, "But God is not like a  pie  or  an apple which can be divided
    into three thirds which  form  one  whole;  if  God  is three persons or
    possesses  three  parts,  He  is  assuredly   not  the  Single,  Unique,
    Indivisible Being which God  is  and  which  Christianity  professes  to
    believe in." (pp. 183-184) 
    Looking at it from another angle,  the  Trinity  designates God as being
    three separate entities - the  Father,  the  Son and the Holy Spirit. If
    God is the Father and  also  the  Son,  He  would  then be the Father of
    Himself because He is His own Son. This is not exactly logical. 
    Christianity claims to be a monotheistic  religion. Monotheism, however,
    has as its fundamental belief that God is One; the Christian doctrine of
    the Trinity - God  being  Three-in-One  -  is seen by Islam as a form of
    polytheism. Christians don't revere just One God, they revere three. 
    This is a charge not taken  lightly  by  Christians,  however.  They, in
    turn, accuse the Muslims  of  not  even  knowing  what  the  Trinity is,
    pointing out that the Qur'an sets  it  up as Allah the Father, Jesus the
    Son, and Mary his mother. While veneration of Mary has been a figment of
    the Catholic Church since 431 when  she  was  given the title "Mother of
    God" by the Council of Ephesus, a closer examination of the verse in the
    Qur'an most often cited by  Christians  in  support of their accusation,
    shows that the designation of Mary  by  the  Qur'an as a "member" of the
    Trinity, is simply not true. 
    While the Qur'an does condemn both trinitarianism  (the Qur'an 4:17) and
    the worship of Jesus and  his  mother  Mary  (the Qur'an 5:116), nowhere
    does it identify the actual three  components  of the Christian Trinity.
    The position of the Qur'an is that  WHO  or WHAT comprises this doctrine
    is not important; what is important is that the very notion of a Trinity
    is an affront against the concept of One God. 
    In conclusion, we see that  the  doctrine  of  the  Trinity is a concept
    conceived entirely by man; there  is  no sanction whatsoever from God to
    be found regarding the matter simply because the whole idea of a Trinity
    of divine beings has no place in monotheism.  In the Qur'an, God's Final
    Revelations to mankind, we  find  His  stand  quite  clearly stated in a
    number of eloquent passages: 
    "...your God is One God: whoever expects  to meet his Lord, let him work
    righteousness, and,  in  the  worship  of  his  Lord,  admit  no  one as
    partner." (Qur'an 18:110) 
    "...take not, with God, another  object  of  worship, lest you should be
    thrown into Hell, blameworthy and rejected." (Qur'an 17:39) 
    "...Because, as God tells us over  and  over  again in a Message that is
    echoed throughout All His Revealed Scriptures: 
    "...I am your Lord and Cherisher: therefore, serve Me (and no other)..."
    (Qur'an 21:92) 
    -- Aisha Brown
Towards better understanding. Until then,
peace and blessings.