It is known that, Augustine of Hippo (354-430 A.D.), renowned Christian theologian, at the time when he was writing his dissertation on the Christian doctrine of the Trinity, was once walking along the seashore when he noticed a small boy pouring seawater into a hole on the ground. On enquiring what he was doing, the boy told him that he was pouring the Mediterranean Sea into the hole. “Don’t be stupid,” replied Augustine, “you can’t fit the sea into that little hole. You are wasting your time.” “And so are you,” replied the boy, “trying to write a book about God.” 
Indeed, this was only a conversation, but it was surely instructive! How grand and how majestic the limitless sea, and how little the hole and its small content of water! How much more magnified is the difference between the hole and the cosmos; and beyond that the infinite difference between creation and its Creator! Can we fully understand this Creator, who not only created the heavens and the earth, but has also left His imprint for us to seek after Him? In the Fourth Century, Gregory of Nazianzen observed “it is difficult to conceive God, but to define Him in words is an impossibility.”  This impossibility to define God, however, does not mean that we should not attempt to know God. The Christian faith gives us the freedom to investigate the truth behind this matter, and we shall endeavour in this regard for the rest of this article.
Similar to other monotheistic faiths, Christianity also professes to believe in the existence of one God. However, Christians also believe that, while there is only one God, He subsists in three persons sharing the same substance. This belief, that the one and only God who exists eternally as three persons (Father, Son and Holy Spirit), is known as the doctrine of the Trinity. So, God is a unity of substance with a plurality of persons. Each person is different, yet they share a common substance. The unity is of His substance (what God is), and the plurality is of God’s persons (how he relates within himself).
The doctrine of the Trinity states two basic propositions that are based on the scriptures. The first is that there exists only one infinite and eternal God who is not limited by time and space, and who has no beginning or end. In this regard, Christians are in agreement with the other historic monotheistic faiths. However, the second proposition is that this God exists in three infinite and eternal persons. The three persons of the Godhead are distinct, but they compose only one infinite and eternal being. This doctrine forms the basis upon which the Christian faith finds its reasonability. In fact, the doctrine of the incarnation, which says that Jesus as God became man (without ceasing to be God), and that He is thus both fully divine and fully human, assumes it. 
How did we arrive?
It is true that the Trinity is one of the great mysteries of the Christian Faith. However, unlike an antinomy or paradox, which is a logical contradiction, the Trinity goes beyond reason, but not against reason.  It is known by divine revelation, and is not the subject of natural theology but of revelation (the way God meets us).  The doctrine of the Trinity is not the product of human imagination or philosophical speculation, or a doctrine that religious leaders have invented to confuse their congregations or non-Christian neighbours. It reflects, in fact, the profoundest experiences and keenest intellects of the early Christian Church. Alister McGrath, a former Oxford University Professor, states that:
“The doctrine of the Trinity wasn’t invented—it was uncovered. It is something implicit within all Christian thinking about God, and all that theologians have done is to make it explicit.“ 
It is usually argued that the Christian concept of the Trinity is too complex. However, we should notice that truth is not always simple. As C. S. Lewis aptly puts it, “If Christianity was something we were making up, of course we could make it easier… We are [here] dealing with facts. Of course anyone can be simple if he has no facts to bother about.” 
The doctrine of the Trinity is the outcome of a process of critical reflection on the patterns of divine activity revealed in Scripture, and is sustained in Christian experience.  The doctrine itself, within the confines of human thought and language, seeks to succinctly formulate who God has revealed himself to be and what He has done. For it goes without saying that only God knows who He is, so also it takes God to reveal Himself and His will for us.
The word ‘Trinity’, as coined by the Latin theologian Tertullian in Second Century, itself comes from the Latin term trinitas, which connects three (tres) with one (unus).  But we need to notice that the idea preceded the word. Early Christians could not avoid the idea of God as Father, Son and Holy Spirit as they reflected on the events surrounding the person of Jesus Christ, and their own experience of Him as Lord. Consequently, the doctrine of the Trinity is not found in a single verse of Scripture alone, but rather it is found in the whole testimony of scripture, as seen particularly in the narrative of salvation (God’s interaction with humanity).
The doctrine of the Trinity is arrived at in much the same way as a scientific theory. A scientific theory, for the most part, is a reasoned explanation of observed, and in some cases unobserved, phenomena in the natural world. In a similar manner, the doctrine of the Trinity is an inductive description of what we observe to be the phenomena of God in the Bible. 
Trinity in the Old and New Testament
The idea of the Trinity is revealed progressively in the Bible. In the Old Testament there are hints to the Trinitarian doctrine, such as Gen. 1:26, where God says, “Let us make man in our image”. We see that the name used for God is “Elohim”, which is in the plural. In Psalm 110, King David says, “The Lord says to my Lord, sit at my right hand.” Who are these two Lords? Likewise, there are many other examples of plural thinking about God. Though the word “Trinity” itself does not appear in Scripture, there are several places in the New Testament where Father, Son, and Holy Spirit are described as working together. Asking Jesus to show them the Father (God) so that they could be satisfied, the disciples were startled by Jesus’ answer, “Whoever has seen me has seen the Father.” (John 14:9) Not long after this, Jesus promised his disciples, “And I will ask the Father, and he will give you another Advocate, to be with you forever. This is the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, because he abides with you, and he will be in you” (John 14:16-17). Similar references to such “triadic formulas” of the Trinity appear in the scriptures time and again.
It is indeed clear in the New Testament that there are three persons described in the experience of the one God of the Jewish confession Shema (“Hear, O Israel: The Lord our God, the Lord is one” Deut. 6:4). But how are they distinct? And how are they still one? And what is their relationship with one another? These questions were at the heart of some of the earliest controversies, questions which led to some of the earliest creeds. Let me now bring our attention to the logic behind the doctrine of the Trinity and see if it in any way adds sense to our perception of who God is.
The Logic of the Trinity
The philosophical law of non-contradiction informs us that something cannot be both true and false at the same time and in the same sense. This is the fundamental law of all rational thought. Does the doctrine of the Trinity violate this law? I wish to argue that it does not. First of all, it is important for us to understand what the doctrine of the Trinity is not. The Trinity is not the belief that God is three persons and only one person at the same time and in the same sense. That would be a contradiction. Rather, it is the belief that there are three persons in one nature. This may be a mystery, but it is not a contradiction. That is, it may go beyond reason’s ability to comprehend completely, but it does not go against reason’s ability to apprehend consistently. To put it in terms of the law of non-contradiction, while God is one and many at the same time, he is not one and many in the same sense. He is one in the sense of His substance, but many in the sense of His persons.
There is little debate that the doctrine of the Trinity is unique to Christianity. The Christian theological term “Trinity” (Tri-unity) does not mean “tritheism” (i.e. tri-theism, “three gods”). There is no evidence of affirmation or worship of three gods in the Bible, or in classical Christian creeds; rather, there is massive evidence against it. Nor does the doctrine of the Trinity rest on any ancient belief system, such as the ancient Egyptian belief in three gods (each existing independently): the Osiris, Isis and Horus, or the Hindu belief in Brahma, Shiva and Vishnu. In fact, the doctrine of the Trinity, in the Christian understanding, is a defence of monotheism against atheism, bitheism, tritheism and polytheism, or any form of idolatry (in Arabic—shirk), not to mention agnosticism.  It is also important to understand that the Trinity is not an explanation of a mystery; it is the articulation of a mystery. It does not teach us how God is; it teaches us what God is. And although I discuss the Trinity with trepidation, there are great spiritual truths that can only be experienced by pondering upon this mystery. 
Have you ever wondered what life was like for God before He created the universe? Suppose God existed alone. For a person to exist alone, when he could cause others to exist and interact with him, would be bad. An eternal divine person must be perfectly good, and that involves being a loving person. A loving person (subject) needs someone to love (object); perfect love is love of an equal, and it has to be expressed mutual.  I cannot love when I am “just by myself”. Focussing only on oneself is egotism, not love. Now, how can God be love if he was solitary in “unitary aloneness” from eternity past? The Bible tells us that “God is love” (1 John 4:8), and that love exists within the eternal and divine communion of the Trinity. It also tells us that the Father loved the Son before the creation of the world (John 17:24). The infinite personal medium through whom this love is communicated is the Holy Spirit, and He is also the one who pours the love of God into our hearts (Romans 5:5). 
Another point is that this attribute of God to love is not dependent on His creation. If God could only start loving after He had created us, then He would be dependent on us. He would not be self-sufficient in His attributes. If He is unitary (a Monad), then love necessarily cannot be one of His attributes, nor can justice or mercy or any other relational attribute. Why? Because He is not in any relationship before he creates.
If God is not relational, how would He have come to the idea to create anything at all? How would He come to the desire to have an extension of His relational attributes beyond the Godhead if relationality is not part of His nature? No, we all believe that God created and He communicates with His creation in one form or another. But this can only be if He is love from eternity and hence there must be some relationality and expression of love within God Himself. That is where the Trinity or some kind of plurality becomes philosophically necessary for God. 
The thought that God was not lonely without us should not discourage us. It is good news that God did not create us out of need, but out of His fullness. Christians know that the Trinity they worship is not a static deity, but rather that there are dynamic relations among the Father, the Son, and the Spirit. It is out of that dynamic, loving fellowship that both creation and salvation have come into being.  We need to take comfort in the fact that God does not depend upon us to express His love. Certainly God takes delight in us and wills to have relationship with us.
It is also in a Trinitarian understanding that God’s transcendence over creation can coexist with His immanence in creation. The real otherness that exists between the distinct persons of the Godhead explains how God can really be other than and therefore transcendent over the creation. Simultaneously, the oneness of the Triune God makes His immanence in creation a reality. The weaknesses of alternative views of God are obvious: either God becomes hopelessly part of His creation, or He is seen as unapproachably remote and distant. The idea of God being part of His creation makes Him a slave of His creation. A spectrum of religions (e.g. Pantheism and New-age) arising from this belief are, not surprisingly, fatalistic. Ironically, in both these cases, the idea of God is functionally impersonal and distant. We can confidently assert that Trinitarian theology provides the rationale behind our understanding of being (ontology) by bringing together the possibility of transcendence and immanence. 
The Implications of the doctrine of the Trinity
The doctrine of the Trinity is not only foundational in the sense that it describes God’s nature, but also it is structural in providing a model for the development of our understanding of the world around us. The doctrine of humanity depends upon the doctrine of the Trinity, and without it our understanding of human-hood would be deficient and would result in the evils of disparity (e.g. Casteism, Racism etc.).  There are evidences that the idea of the sanctity of human life in Western society developed as a result of the Christian doctrine of God as Trinity.  It affirmed the value of persons and showed that to be human is to be personally related to a personal God and to other human beings. It ultimately led to the establishment of a vastly higher standard of care and respect for all people. The doctrine of the Trinity advocates the reality, the personality and the value of the individual in sharp contrast to the idea that an individual is an impersonal entity doomed forever in meaninglessness.
The doctrine of the Trinity also enables us to think properly about the physical nature of the world we live in. The world is neither merely mechanical nor biologically determinist. Its existence is contingent upon God who created it, who sustains and who will ultimately transform it. The doctrine of the Trinity gives us hope and tells us that the same God who has created the world has entered into it in Christ, lives within it through the Spirit and will redeem it out of its fallen state. The suffering of the Son on the cross is perhaps the most challenging dimension of Trinitarian theology, yet it is also the richest and most inspirational statement about the way God has dealt with our sin and suffering. The implication of the Triune God is not individualism but social relationship.  The Kingdom of God is a social reality. 
The purpose of this article has been to offer a framework in explanation of the doctrine of the Trinity. Let me reiterate that it is not the doctrine of the Trinity that underlines the Christian faith, but the living God whom we encounter through Jesus Christ in the power of the Holy Spirit— the God who is Triune. Do you and I understand the Trinity well? No. We struggle to expand our finite mind to encompass such a thought. The God who resides outside our dimensions cannot be exhaustively comprehended. Though He stands beyond us in mystery, He can be known by His gracious revelation given in finite categories and conditions that have meaning for us as finite beings. Nevertheless, what God has revealed of Himself is fully sufficient for us to know and to love Him. 
 Alister McGrath, Understanding Jesus (Grand Rapids: Zondervan, 1990), pp.30-31.
 St. Gregory Nazianzen, Theological Orations 24:4
 The Apologetics Study Bible (Nashville: Holman Bible Publishers, 2007), p.1459.
 Norman L. Geisler, Baker Encyclopedia of Christian Apologetics (Grand Rapids: Baker Academic, 2007), p.732.
 Ibid, p.730.
 Alister E. McGrath, Understanding the Trinity (Grand Rapids: Academie Books, 1988), p.148.
 C. S. Lewis, Mere Christianity (New York: Macmillan, 1953), p.145.
 Alister E. McGrath, Christian Theology: An Introduction (Oxford: Blackwell, 1994),p.249.
 Brian Edgar, The Trinity and life in God, “The Message of the Trinity” (2010) at http://brian-edgar.com/theeology/god-as-trinity/the-trinity-and-life-in-god/, accessed on 13/05/11
 Source: http://www.answering-islam.org/Trinity/beckwith.html, accessed on 09/04/2011.
 Source: http://www.answering-islam.org/Hahn/trinity.htm, accessed on 11/05/11.
 Source: Slice of Infinity, http://dev.rzim.org/resources/read/asliceofinfinity/, accessed on 08/05/2011.
 Richard Swinburn, Was Jesus God (Oxford: Oxford University Press, 2008), p. 18.
 L. T. Jeyachandran The Trinity as a Paradigm for Spiritual Transformation in Ravi Zacharias Beyond Opinion (Chennai: RZIM Educational Trust, 2007), p.241.
 Source: http://answering-islam.org/Trinity/love.html, accessed on 11/05/11.
 Source: Slice of Infinity, http://dev.rzim.org/resources/read/asliceofinfinity/todaysslice.aspx?aid=8689, accessed on 12/05/11.
 L. T. Jeyachandran The Trinity as a Paradigm for Spiritual Transformation in Ravi Zacharias Beyond Opinion (Chennai: RZIM Educational Trust, 2007), p.240.
 Source: http://thedivinecommunity.blogspot.com/2008/04/practical-implications-of-trinity.html, accessed on 12/05/11.
 W.E.H. Lecky, History of European Morals from Augustus to Charlemagne (New York: George Braziller, 1955).
 Jürgen Moltmann, The Trinity and the Kingdom of God (London: SCM Press Ltd., 1981), p. 19.
 Joseph Oomen, “The Concept of Trinity and Its Implication for Christian Communication in India Context”, Bangalore Theological Forum 34 (2002), pp. 75-82.
 Scott Horrell, “In the name of the Father, Son, and Holy Spirit: Towards a Transcultural Trinitarian Worldview”, Trinitarian Study (April 2009).