A distinguished Catholic theologian once remarked, 'Christians are convinced they have a message for all men, but they do not have a monopoly of the truth.' That there is one God is no monopoly of the Christian faith. Amongst those absolutely committed to it are the Jewish people and the followers of Muhammad. 'Hear, O Israel, the Lord our God is one Lord'(Deut. 6:4) or as the Holy Qur-an puts it, 'God is one God, Glory be to him.'(Sura IV.171)
When the Christian talks of three Persons in one God, the Jew and the Muslim begin to be fearful. 'Far exalted is He above having a son', says the Qur-an, 'O people of the Book, commit no excesses in your religion; nor say of God aught but the truth. Christ Jesus the son of Mary was no more than an apostle of God.'(Sura IV.171) Muhammad never saw himself as anything but a prophet, as was Moses.
It would be simpler if Christians could say the same of Jesus and abandon their doctrine of the Trinity. But this is where their conviction comes in that they have a message for all the world. For them the tradition of their received Scriptures is that Jesus is in very truth the Son of God. Christians read of it in many places, notably in the Gospel of John. When Jesus said 'Before Abraham ever was, I am', his hearers picked up stones to throw at him - the penalty for blasphemy (John 8:59).
Much of John's Gospel is a kind of meditation on the relation of Jesus to the Father. Of course 'Son of God' was a familiar enough term and was applied to Israel as a whole or to a devout Israelite individual. But in the three first Gospels, when the term is used of Jesus it seems progressively to indicate a much closer union of Jesus with the Father; his is a unique and supernatural sonship as testified at his baptism and transfiguration. This understanding was developed more fully in Paul's epistles. Jesus in his resurrection is revealed as pre-eminently the Son of God.
This development comes to a head in John's Gospel. Jesus is able to pray 'Father, glorify me with that glory I had with you before ever the world existed' (Jn 17:5). To Philip he can say, 'Anyone who has seen me has seen the Father ... Do you not believe that I am in the Father and the Father is in me?' (Jn 14:9) Many other passages point to the same close union, though Father and Son remain somehow distinct. The same may be said of the Spirit, though we have no time here to go into it.
It was this living conundrum that led Christian thinkers to formulate the teaching of the council of Nicaea in the year 325 AD: 'We believe in one God: the Father All-sovereign ... and in one Lord Jesus Christ ... of one substance with the Father ... and in the Holy Spirit' - the creed ever since of all Christian churches.
You may ask what sense we can make of it. Three Persons, One God. Perhaps one answer might come if we think for a moment about the different ways in which things may be present one to another. Two objects - say, two tables, standing next each other - are present to each other. But that is nothing by comparison with the way living creatures can be present. They commune with one another. But even here there are different levels. My talking to a stranger shares nothing of the closeness between two friends, and this in turn nothing of the intimate union of two lovers.
The thing to notice is that as the degree of self-awareness increases, so does the intimacy made possible. The measure of closeness goes hand-in-hand with the degree of selfhood. Two tables have no self-awareness; and those engaged in only a passing exchange do not reveal themselves as fully as do two lovers. Let us project this to infinity. When union reaches total unity, Father, Son and Holy Spirit are revealed as (we might possibly say) absolute polarities of perfect love.
The doctrine of the Trinity, so far from belittling the unity of God, reveals the richness of his Being. Jesus, the Son made flesh, reveals God's life - a life of such glory and mutual love that nothing in our experience can compare, the beauty of the one God, to whom be glory for ever.