I certainly can't exhaust the mystery of the Holy Trinity in a brief homily. Volumes have been written on the subject, and they only scratch the surface. The one insight that comes to me--I hope, in answer to my prayers--tonight is to hold up our trinitarian language about God--the language the Christian community uses to talk about God--as a sort of "canary in the coal mine." (This image, of course, comes from the old practice of coal miners, realizing that canaries are more sensitive to dangerous gases than humans, taking a canary down into the shaft with them to serve as an early warning system for potential hazards.)
Even though passages like tonight's gospel from Matthew 28 (the Great Commission) use language that seems compatible with full-blown trinitarian theology, it is a mistake to read that theology back into those passages. The fact is, it took the Church more than 400 years to sort out how we speak of God in trinitarian terms--or, more precisely, how we don't speak of God in trinitarian terms, because this is one area of theology that is done pretty much by elimination: We discerned/discovered ("were told," actually, by the Holy Spirit, but that's a faith statement) that "this" is not how we can speak about God, and "that" is not how we can speak about God, etc. etc. So, whatever is left after eliminating all the demonstrably false articulations of the mystery is what's available to us. That becomes the raw material for our theologizing. And what we are left with is "one God in three Persons": Father, Son, and Holy Spirit.
One of the things our trinitarian language for God reminds us of is that the God we worship is not a concept, not an abstraction, not merely the idea of a Supreme Being. Rather, we worship a particular God. This is something our polytheistic forbears (even our polytheistic Hebrew forbears; Psalm 29 tonight mentions "gods") could see more easily than we can through our staunchly monotheistic lenses. In the Daily Office, we've been reading through II Kings. Recently, we read how, after the Assyrians deported the population of the Northern Kingdom, the settlers sent in to replace them were beset with plagues. The Assyrians just figured they didn't know how to properly worship the (particular) God of that land, so they sent back a priest from among the exiles to teach them how to do it! And when the LORD appears to Moses in the burning bush (tonight's reading from Exodus), he stresses the particularity of his identity: "I am the God of your fathers; the God of Abraham, the God of Isaac, and the God of Jacob." It wasn't just any God that Moses was dealing with, not simply a deity, or even The Deity. No, it was the God of Abraham, Isaac, and Jacob, and, as St Paul would add, the God and Father of our Lord Jesus Christ. This is the God, and no other, whom Christians worship and serve.
So, the language of our trinitarian theology--the words we use to talk about God, and, by implication, the words we don't use to talk about God--serves as a boundary, a fence. It reminds us of the particularity of the God who is the object of Christian worship, because, left to our own devices, we will very quickly generalize our experience of God into vague abstractions. I know that, to many, trinitarian theology seems arcane and fussy, something professional theologians may get excited about, but which doesn't have any real impact on the real lives of ordinary Christians. Yet, this is precisely where it functions as the canary in the coal mine, because whenever we get the urge to jump the trinitarian fence, the canary keels over. Church history is full of examples of individuals and groups ignoring or contradicting orthodox trinitarian language. When that happens, nothing but turmoil and schism result. It never ends well.
Of course, ultimately the Trinity is not a doctrine that we must understand, but a God whom we must worship. So let us get on with that job, and worship the glorious and undivided Trinity.