It may be true that the 'mystery' of the Trinity is not a mystery of the Agatha Christie kind, not something hidden to be puzzled out, but something revealed to be explored ever more deeply. But the fact is that we do find Trinitarianism baffling, and it can be a source of great anxiety; and with good reason, because it's so easy when talking about the Trinity to say the wrong thing.
We can easily end up either believing in three different Gods, or in saying that the three Persons of the Trinity are nothing more than just three ways of talking about the one God. So if talking about the Trinity is so tricky, so full of snares for the unwary, wouldn't it be better just not to bother?
On the contrary, today's Gospel shows that we have no choice but to talk about the Trinity. It is not an optional extra to be added on to our faith in Christ, but the very heart of our religion: there is no God for us to believe in but the Triune God, no Christ to follow but the incarnate Word, no life for us to live but the divine life breathed into us by the Holy Spirit; and there is no message for us to preach but baptism in the name of the Father, the Son and the Holy Spirit.
This is the command that Christ gives us at the end of Matthew's Gospel; his last testament to us amounts to nothing less than a share in his own mission.
Throughout his Gospel, Saint Matthew presents the life and mission of Christ as the re-creation of Israel, the gathering around himself of the chosen People of God, a community of saints. This is what is implied by the word 'disciples', those gathered by Christ to be the new Israel for a new age. So in commissioning the eleven -- and through them, us -- to make disciples of all the nations, he is giving us a share in his work, the work of the Word made flesh, the work of the Triune God.
The first Israel was set apart from the nations, uniquely privileged and distinguished under the leadership of Moses, and today's first reading is typical of the Book of Deuteronomy in celebrating the distinction of God's chosen people. But Israel always looked forward to the time when God would lead the Gentiles to join her, would make them one flock with Israel at 'the end of the age'.
Now, in his risen body, Christ shows his disciples that the time has come, the new age has dawned; and it is their task to draw the Gentiles, the nations, into the People of God. When he tells us that
All authority in heaven and on earth has been given to me,
he is claiming to be the fulfilment of the Book of Daniel, in which
One like a son of man came to the Ancient of Days and was presented before him. And to him was given dominion and glory and kingdom, that all peoples, nations and languages should serve him.
Jesus Christ, then, has raised humanity to the level of the divine. And because of who Christ is and what he has done -- because of the incarnation and resurrection -- the old age is over, and it is time for all the nations to be gathered into the People of God. Our identity and our mission as Christians is inseparably intertwined with the identity and mission of Christ.
At the beginning of his Gospel, Matthew tells us that Christ is Immanuel, God-with-us. Jesus himself re-asserts this claim himself at the end of the Gospel when he says
I am with you always.
In the Greek this 'I am' is an emphatic ego eimi -- not just 'I am', but I AM, the divine name that God reveals to Moses from the burning bush.
In the life and work of the Church, which is the new Israel, the Holy Spirit brings Christ's presence among us; when we allow the Spirit to dwell in our hearts, and so by the Spirit of God are incorporated into the People of God, we are raised up with Jesus Christ, Son of Man and Son of God, to dwell with the Father. Christ tells us today that our mission as the Church is a Trinitarian one; and so our lives become Trinitarian too, as we share in the life of the Triune God.