God is plural not singular in nature
T    H    E      T    R    I    N    I    T    Y
If God is truly personal he must have an eternal relationship with 
another eternal person. How could a singular God be 
personal if there is no one else to eternally relate too, to love? 
There must be an eternal plurality if God is personal. 
This is both Biblical and rational.
Genesis 1:26: 3:22
"Let us make man in our image after
our likeness...Behold, the man has become as one 
of us, knowing good and evil" 
The Personal God 
When we say that God is personal we mean that he has all the qualities that give us the ability to relate to each other. God is a thinking, feeling, and willing eternal person. Biblically speaking, we are created in the image of God. If God is a singular personal being then only the creation of another personal being would actualize his potential to be relational. God, then, becomes relational and experiences love only after he creates another person. But this doesn't make sense because whatever God is, he is intrinsically. God cannot be dependent upon a created, finite, temporal man, nor angel, in order to be personal. There must exist another uncreated, infinite, and eternal person in order for God to be truly personal being.
FATHER
SON        The Spirit is not the Son      SPIRIT
Ultimate eternal reality is personal and plural
Where an eternal relationship of love, communication, unity with diversity has always 
existed; and Father, Son, and Holy Spirit are one.
GOD
The meaning of "one" 
Deuteronomy 6:4, "The Lord your God is one Lord".  The word for God used here is "eloheem", a plural form for God. The word "one" can be interpreted as "only" or as "united", depending upon the context. Because "eloheem" is used, this verse should be interpreted as meaning the unity of God--the Trinity. It is the only way that to rationally explain, "Let us make man in our image". 
Greek Philosophy 
The only contradiction to pluralism in God is the concept of Platonic "Perfect Being" and Aristotle's "Unmoved Mover". This God is absolutely immovable, immutable, and timeless, and is the exact opposite of the Biblical concept of God. See Dynamic Free Theism.
Some have argued that Jesus did not say he was equal to God. The fact is, Jesus never said he was the Father and it is possible to be subordinate to the Father, just as the Spirit is subordinate, and still be an eternal being. He was accused many times of making himself equal to God and he never disputed the charge. One would have to believe that Christ used the misconception of his enemies to accomplish his purposes by allowing them to believe a lie, which would be an act of deception on his part. Below are the scripture passages that pertain to this important subject. As a whole they seem to be very clear and supportive of the case for the Trinity.
The Son is not the Father
The Father is not the Spirit
John 1:1-3  In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God; all things were made through him, and without him was not anything made that was made.

John 1:10-14  He was in the world, and the world was made through him, yet the world knew him not. He came to his own home, and his own people received him not. But to all who received him, who believed in his name, he gave power to become children of God; who were born, not of blood nor of the will of the flesh nor of the will of man, but of God. And the Word became flesh and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father.

John 5:15-21  The man went away and told the Jews that it was Jesus who had healed him. And this was why the Jews persecuted Jesus, because he did this on the Sabbath.
     But Jesus answered them, "My Father is working still, and I am working."
     This was why the Jews sought all the more to kill him, because he not only broke the Sabbath but also called God his Father, making himself equal with God.
     Jesus said to them, "Truly, truly, I say to you, the Son can do nothing of his own accord, but only what he sees the Father doing; for whatever he does, that the Son does likewise. For the Father loves the Son, and shows him all that he himself is doing; and greater works than these will he show him, that you may marvel. For as the Father raises the dead and gives them life, so also the Son gives life to whom he will.

John 8:53-59  Are you greater than our father Abraham, who died? And the prophets died! Who do you claim to be?"
     Jesus answered, "If I glorify myself, my glory is nothing; it is my Father who glorifies me, of whom you say that he is your God. But you have not known him; I know him. If I said, I do not know him, I should be a liar like you; but I do know him and I keep his word. Your father Abraham rejoiced that he was to see my day; he saw it and was glad."
     The Jews then said to him, "You are not yet fifty years old, and have you seen Abraham?"
     Jesus said to them, "Truly, truly, I say to you, before Abraham was, I am." (Exodus 3:14  God said to Moses, "I am who I am.")
     So they took up stones to throw at him; but Jesus hid himself, and went out of the temple.

John 10:24-38  So the Jews gathered round him and said to him, "How long will you keep us in suspense? If you are the Christ, tell us plainly."
     Jesus answered them, "I told you, and you do not believe. The works that I do in my Father's name, they bear witness to me; but you do not believe, because you do not belong to my sheep. My sheep hear my voice, and I know them, and they follow me; and I give them eternal life, and they shall never perish, and no one shall snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. I and the Father are one."
     The Jews took up stones again to stone him.
     Jesus answered them, "I have shown you many good works from the Father; for which of these do you stone me?"
     The Jews answered him, "It is not for a good work that we stone you but for blasphemy; because you, being a man, make yourself God."
     Jesus answered them, "Is it not written in your law, 'I said, you are gods'? If he called them gods to whom the word of God came, and scripture cannot be broken; (Psalm 82) do you say of him whom the Father consecrated and sent into the world,'You are blaspheming' because I said, I am the Son of God"?

John 14:6-11  Jesus said to him, "I am the way, and the truth, and the life; no one comes to the Father, but by me. If you had known me, you would have known my Father also; henceforth you know him and have seen him."
    Philip said to him, "Lord, show us the Father, and we shall be satisfied."
    Jesus said to him, "Have I been with you so long, and yet you do not know me, Philip? He who has seen me has seen the Father; how can you say, 'Show us the Father'? Do you not believe that I am in the Father and the Father in me? The words that I say to you I do not speak on my own authority; but the Father who dwells in me does his works. Believe me that I am in the Father and the Father in me; or else believe me for the sake of the works themselves.

John 17:1-5  When Jesus had spoken these words, he lifted up his eyes to heaven and said, "Father, the hour has come; glorify thy Son that the Son may glorify thee, since thou hast given him power over all flesh, to give eternal life to all whom thou hast given him. And this is eternal life, that they know thee the only true God, and Jesus Christ whom thou hast sent. I glorified thee on earth, having accomplished the work which thou gavest me to do; and now, Father, glorify thou me in thy own presence with the glory which I had with thee before the world was made.

John 17:20-26  "I do not pray for these only, but also for those who believe in me through their word, that they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us, so that the world may believe that thou hast sent me. The glory which thou hast given me I have given to them, that they may be one even as we are one, I in them and thou in me, that they may become perfectly one, so that the world may know that thou hast sent me and hast loved them even as thou hast loved me.
     Father, I desire that they also, whom thou hast given me, may be with me where I am, to behold my glory which thou hast given me in thy love for me before the foundation of the world. O righteous Father, the world has not known thee, but I have known thee; and these know that thou hast sent me. I made known to them thy name, and I will make it known, that the love with which thou hast loved me may be in them, and I in them."

John 20:24-31  Now Thomas, one of the twelve, called the Twin, was not with them when Jesus came. So the other disciples told him, "We have seen the Lord." But he said to them, "Unless I see in his hands the print of the nails, and place my finger in the mark of the nails, and place my hand in his side, I will not believe."
     Eight days later, his disciples were again in the house, and Thomas was with them. The doors were shut, but Jesus came and stood among them, and said, "Peace be with you." Then he said to Thomas, "Put your finger here, and see my hands; and put out your hand, and place it in my side; do not be faithless, but believing."
     Thomas answered him, "My Lord and my God!"
     Jesus said to him, "Have you believed because you have seen me? Blessed are those who have not seen and yet believe."Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.

Mark 2:3-12  And they came, bringing to him a paralytic carried by four men. And when they could not get near him because of the crowd, they removed the roof above him; and when they had made an opening, they let down the pallet on which the paralytic lay. And when Jesus saw their faith, he said to the paralytic, "My son, your sins are forgiven." Now some of the scribes were sitting there, questioning in their hearts, "Why does this man speak thus? It is blasphemy! Who can forgive sins but God alone?"
     And immediately Jesus, perceiving in his spirit that they thus questioned within themselves, said to them, "Why do you question thus in your hearts? Which is easier, to say to the paralytic, 'Your sins are forgiven,' or to say, 'Rise, take up your pallet and walk'? But that you may know that the Son of man has authority on earth to forgive sins" -- he said to the paralytic-- "I say to you, rise, take up your pallet and go home." And he rose, and immediately took up the pallet and went out before them all; so that they were all amazed and glorified God, saying, "We never saw anything like this!"

Mark 14:60-65  And the high priest stood up in the midst, and asked Jesus, "Have you no answer to make? What is it that these men testify against you?" But he was silent and made no answer. Again the high priest asked him, "Are you the Christ, the Son of the Blessed?"
     And Jesus said, "I am; and you will see the Son of man seated at the right hand of Power, and coming with the clouds of heaven."
     And the high priest tore his garments, and said, "Why do we still need witnesses? You have heard his blasphemy. What is your decision?" And they all condemned him as deserving death. And some began to spit on him, and to cover his face, and to strike him, saying to him, "Prophesy!" And the guards received him with blows.

Mat 3:16-17  And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, "This is my beloved Son, with whom I am well pleased."

Mat 28:18-20  And Jesus came and said to them, "All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age."
The Biblical 
the Trinity 

Francis Schaeffer
"Let us think of the Nicene Creed--three Persons, one God...If this were not so, we would have had a God who needed to create in order to love and communicate. In such a case, God would have needed the universe as much as the universe needed God. But God did not need to create; God does not need the universe as much as the universe needs him. Why? Because we have a full and true Trinity. The Persons of the Trinity communicated with each other, and loved each other before the creation of the world."
 Case for