Every preacher looks forward to the chance of preaching on the Trinity.
It is just as well this homily is in written form: it's difficult to speak with tongue firmly placed in cheek. There is no doubt that preaching on the Trinity is not straightforward. What worries the preacher is saying something wrong in error. A vague use of three musketeers language ('one for all and all for one') and the preacher hopes to have avoided the pitfalls. I suspect that many preachers carefully destroy any notes they have made for a homily on the Trinity. Unfortunately these particular notes may exist in cyberspace for eternity.
Perhaps, however, that sense of trepidation needs to be confronted head on. The Trinity is at the heart of Christian faith: it is not something to be glossed over. There is a tendency to say 'the Trinity is a mystery' and then move on. For the limited human intellect, aspects of God in this life will always be opaque, but we should keep the mysterious as limited as possible. We should always be at pains to know the knowable. What is not a mystery should not be made mysterious by us.
Trinitarian theology is complicated. It must be so for a reason. Hopefully, its very complications bring us closer to God. Other monotheistic faiths look at the Trinity and think Christianity seems suspiciously polytheistic. To explain otherwise is not straightforward. Hopefully, we have given ourselves this cross to bear for good reason.
A first good reason is that Scripture is full of references to the Father, Son, and Holy Spirit, with today's gospel reading being as good an example as any. If Scripture talks of Father, Son and Holy Spirit then so do we. That is not to say a defined doctrine of the Trinity is found Scripture, but certainly the divine vocabulary of the Trinity is.
Today we are told that Jesus still has many things to say but they are too much for the disciples at this point. The crucifixion is still in the future. The Holy Spirit is still to come. The Spirit will complete the truth that Jesus began. The truth to be learned is that the crucifixion is not a tragedy but a victory that opens for us a way to salvation.
If we had no knowledge of Jesus and possessed no bible, would we come up with anything like the doctrine of the Trinity? I doubt it. It looks overly convoluted when taken out of context. Then, when we look at today's gospel reading, it is difficult to think how things could be otherwise.
God does not want to leave us where we are. God wants us to find our way to Him. God gives His Son for that purpose. Jesus has a lot to tell us, but we are not capable of possessing that much knowledge of the divine. We need a Holy Spirit to bring us to a full knowledge of God. Just as the disciples saw the crucifixion as a defeat rather than a victory, so we don't really understand how we are saved. We need the Holy Spirit to bring us to full knowledge.
Looked at this way, it seems obvious that we do need a Father, Son and Holy Spirit to bring us to our salvation. The Trinity is the heart of our faith insofar as we believe that the one God is truly present to us for our salvation in Jesus Christ and the Holy Spirit. The Trinity lies behind the conviction that was manifest in Jesus: a creative, responsive love is the basic reality of the universe.
It is through encountering the Trinity that we come to knowledge of God, and through the Trinity, God achieves his purpose of our Salvation. In talk of the Trinity we recognise in history and in our own lives the God we meet in the Bible.
We could have come up with something other than three persons in one God. But in the end the Trinity is a way of understanding God that brings us to some kind of appreciation of complexities of the divine: one God who is bringing us to our salvation. The Trinity is our way of understanding God and how he loves us.