Above all else Christianity reveals who God is. Based directly on Jesus’ own words about his eternal unity with the Father and the Holy Spirit, the Church has always taught that God is one God, but three distinct persons. The three divine persons are the One Triune God.
The existence of the one God has been known to faith and reason throughout history. God chose to reveal himself as one Lord to the people of Israel, to teach them that he is the creator of all things and the single, true and exclusive object of worship. However, the Old Testament gives glimpses of personal distinctions in the one God. An example is the use of the plural pronoun ‘us’ at the creation of human beings: ‘Then God said, “Let us make man in our image, after our likeness.” ‘Gen 1:26
In the New Testament, when God the Son becomes man, he openly reveals the persons in God. First the relationship between Father and Son is revealed: “No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known.” (Jn 1:18) He then reveals their relationship of the Father and the Son with the Holy Spirit: “When the Paraclete comes, whom I shall send to you from the Father, the Spirit of truth who issues from the Father, he will be my witness.” (Jn 15:26 NJB)
All Christian life begins with Baptism in the singular name of the three divine persons, following Jesus’ command:“Make disciples of all nations; baptise them in the name of the Father and of the Son and of the Holy Spirit.” (Mt 28:19-20 NJB)
The Trinity is the source and centre of the Christian faith. The divine persons introduce each section of the Creed.: I believe in God, the Father Almighty; I believe in Jesus Christ his only son, our Lord.; I believe in the Holy Spirit.
God alone reveals the doctrine of the Trinity. Human reason can know that there is a God; we cannot know God as he knows himself except from what God has revealed. The fact that Jesus Christ reveals the relationship of Father, Son and Holy Spirit, tells us that these are distinct divine persons. Each divine person can properly say ‘I’, as when Jesus says “I and the Father are One” (Jn 10:30). Jesus also uses a masculine personal pronoun (translated ‘he’) of the Holy Spirit when he says, “the Counsellor, the Holy Spirit, whom the Father will send in my name, he will teach you all things” (Jn 14:26).
Nevertheless, the relations within the Trinity differ from those among human persons. Our relations are changeable, and established over time. By contrast, the relations of the divine persons are the very being or ‘substance’ of God. They are eternal and unchanging. This oneness of being along with the distinction of divine persons is expressed in the Trinitarian formula affirmed by the early Church: The Trinity is one substance, three persons.
It is impossible to imagine the Trinity correctly. No one representation of the Trinity could ever be adequate to express every aspect of the Trinity, which exceeds any created thing. So the best way to represent the Trinity is to try to avoid just one, single image, but to use a variety of images to emphasise distinct but complementary aspects of the Godhead. Certain images are good at emphasising the unity of the Godhead. An example of such an image is the Irish Shamrock, a single plant with three leaves, which was used by St Patrick when preaching the gospel to the pagan tribes of Ireland in the fifth century.
While all images eventually mislead due to their materiality there is no contradiction in speaking of three persons in one God. That which is three in one sense can, at the same time, be one in another sense. For example, the three sides of a triangle are three insofar as they are sides, but are inseparably one insofar as they are a triangle. So while the Trinity undoubtedly exceeds human comprehension, the teaching about the Trinity that has been drawn from revelation, namely that God is ‘one substance, three persons’, is not, in itself, inherently contradictory.
Since the Trinity is the central revelation of Christianity about God, the practical impact of the Trinity is intimately linked to every aspect of our lives as Christians. To select one example, the revelation that God is a Trinity (rather than, for example, God as a singular person, if this were possible) underlines the foundational importance of interpersonal relationships comprising two key elements: a perfect unity of love, yet without one person absorbing the others or annihilating their personal identity. These principles are reflected, for example, in the institution of Christian marriage, the notion of Christian friendship and a vision of society in which properly distinct institutions work in mutual harmony. One related idea is the Catholic political principle of subsidiarity. Another practical impact of the Trinity on our lives is that this revelation can give us hope for our future happiness in heaven. In particular, the revealed life of the Trinity answers two deep and conflicting human fears – loneliness and loss of identity. Just as the persons of the Trinity are united but distinct in their communion, in heaven we shall be neither isolated nor absorbed. Indeed, in relationship with God in heaven we shall also truly be ourselves and know ourselves.
God does not want us to relate to him merely as creatures to their Creator, but to enjoy intimate friendship with him by sharing the divine life of the Trinity. This is why all Christian belief is Trinitarian (for example, the Creed); all sacraments are Trinitarian (for example, Baptism in the name of the Trinity); all Christian life is directed towards union with the Trinity (for example, the virtue of charity) and all Christian prayer is Trinitarian (for example, the Sign of the Cross).