“I still have many things to say to you, but you cannot bear them now.  When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own, but will speak whatever he hears, and he will declare to you the things that are to come.  He will glorify me, because he will take what is mine and declare it to you.  All that the Father has is mine. For this reason I said that he will take what is mine and declare it to you.”
In the Church calendar, Trinity Sunday follows immediately after Pentecost and so is a moveable feast day, this year falling on June 16. It is unique among the feasts of the Church as it doesn’t celebrate an event (e.g. Christmas, Easter, Pentecost) or an individual such as a saint. Instead this feast focuses on the doctrine of God—the teaching that God is three-in-one; Father, Son and Holy Spirit, three persons who are one God.
The temptation is to try and explain the doctrine in detail so people can understand it. But the Trinity isn’t a set of propositions to be explained but a mystery to wonder at. Some might think this is a cop-out—but seriously, if we could explain God completely, and understand God exhaustively, would God really be God?
Or think of it in simpler terms. Consider someone you’ve known for a while, maybe a spouse, parent or friend. You could say with confidence that you know them well, and no one would dispute that. But the implicit understanding is that you don’t know everything about them—there continue to be things you learn about each other as time goes on yet you know them well enough to be in a close relationship.
It’s like that with the Trinity. From Scripture and the Creeds we can know about God e.g. that God’s name is Father, Son and Holy Spirit, the name in which the Church baptizes. We know God created everything that exists, and that God loves His whole creation.
But the important next step is to move from knowing about God, to knowing God in relationship—God has claimed us as His children in Holy Baptism, and Jesus comes to us in Holy Communion. We can go to God in prayer, worship Him in service, and read God’s message to us in Holy Scripture.
The Gospel text for Trinity Sunday reminds us of God’s desire for us not just to know about Him but to be in relationship with Him. Doctrine isn’t an end in itself, but exists to point and urge on to relationship and life with God.