The doctrine of the Trinity is not mysterious. The doctrine simply concedes, as honestly and candidly as it can, the fact that God is mysterious. As Herbert McCabe says (in his essay, "Aquinas on the Trinity," published in New Blackfriars), it is true that when we're speaking of God we're "trying to talk of what we cannot talk of, trying to think of what we cannot think." But that doesn't mean we're dealing in contradictions or gibberish. instead, it means that we're giving witness to something that simply doesn't fit within reality as we know it. For Christians, "God" is the name for whatever it is that gives us our reality in all of its goodness, and gives it to us in such a way that we are both drawn out toward a fulfillment that nothing in reality can give us—and eager to share that fulfillment with everyone and everything.
Truth be told, we don't know anything about God. The God we know, the God we are known by, cannot be known. Of course, we know who God is, and that God is. But our knowing of God is a knowing that trespasses knowledge and understanding, and precisely for that reason makes us wise (Eph. 3.19). Strictly speaking, we can't know anything about God, because God is not a thing. Again, McCabe makes this unusually clear:
God must be incomprehensible to us precisely because he is creator of all that is and, as Aquinas puts it, outside the order of all beings. God therefore cannot be classified as any kind of being. God cannot be compared or contrasted with other things in respect of what they are like as dogs can be compared and contrasted with cats and both of them with stones or stars. God is not an inhabitant of the universe; he is the reason why there is a universe at all. God is in everything holding it constantly in existence but he is not located anywhere, nor is what is God located anywhere in logical space. When you have finished classifying and counting things in the universe you cannot add: "And also there is God." When you have finished classifying and counting everything in the universe you have finished, period. There is no God in the world.
So, God is mystery. And, unsurprisingly, talking about God is exceedingly difficult. In McCabe's words, "There is nothing especially odd or irrational about [the doctrine of the Trinity]. It only seems shocking to those who expect the study of God to be easy and obvious, a less demanding discipline than, say, the study of nuclear physics." All to say, we should not despise the difficulty that speaking and thinking about God forces on us. In fact, we should delight in it. In the presence of God, reason does not so much break down as bow down, bending in glad acceptance of its givenness, even while it is happily straining against its limits. Preaching is a strain in just this sense. The preacher strains to say something about the Mystery that creates and sustains and perfects all things, and in that straining she either becomes transparent to it or blocks the light, casting a revealing shadow. Either way, she bears witness.
Every sermon should identify God as Trinity. If it fails to do that, then it has failed to get the gospel said, and so cannot really be called a "preaching" in any meaningful sense. Some sermons (perhaps at least one every season of the Christian year?) need to spell out the basic shape of the doctrine and provide some interpretation of it.
We may fear that such sermons are impractical, irrelevant, or merely academic. If so, we’ll be tempted to make them seem more practical by construing the divine life as a kind of model for living lives of mutual enrichment. There‘s truth to that idea, no doubt. But we need to return to the deeper truth: we're not called to live before God as subordinates but blessed to live in God as equals. More than that, we're blessed to become God, one with the Son in the Father's love, made by the Spirit constituent parts of God's being. Here's how McCabe puts it:
[Because of the divine missions] the life of the Trinity becomes available to us... both in the sense that we know of it, believe in it, and in the sense that we belong to it. These are of course the same thing. It is because we share in the Holy Spirit through faith and charity and the other infused virtues that we are able to speak of the Trinity at all. It is not therefore adequate to speak of God's redemptive act as an opus ad extra. It is precisely the act by which we cease to be extra to God and come within his own life.
As my pastor said this morning, the Trinity is not a model for living but the reality in which we live. And good sermons, accordingly, unsettle us—later, remember that I warned you—by reminding us that the God about whom and to whom and for whom we're speaking is not a God of our own making, not a God who can be used, and for those very reasons is a God who can be enjoyed.
McCabe says the whole of the doctrine of the Trinity is embedded in the Father's confession over the Son at his baptism: "This is my beloved Son in whom I am well pleased" (Mt. 3.17). McCabe knew this because Aquinas knew it. And Aquinas knew it because the Fathers knew it. And the Fathers knew it because this was essential to the apostles' teaching, which is why the pattern appears at many places in Scripture.
You can see it, for example, in Eph. 3.19 (the passage I mentioned earlier): God the Spirit is the unknowable love the Son receives from the Father and gives to us; and because that Spirit is unstintingly given and received, Jesus both is the fulness of God for us and shares that fulness with us. You can see it in Tit. 1.2: creation exists, and is assured of a glorious future, because God (the Father) promises God (the Son) to give us God (the Spirit). You can see it in Phil. 4.19: the Spirit is the "wealth" the Father lavishes on the Son (and so on us, as his co-heirs); receiving that inheritance—receiving God—we receive all good things, and so are fully satisfied, blessed exactly as God is. All of these passages, and others like them, testify to the same wonder. And that wonder, if we let it, will not only change our preaching, it will also change us and those we're bound to care for in ways we cannot even begin to imagine.
That sounds like so much bullshit, I know. But listen to this—from McCabe of course:
To take the doctrine of the Trinity seriously is to say that divinity is now found in people, that they create their own destiny in virtue of his divinity. It is to say that the picture of the prearranged plan worked out by God up there, to which we must conform, is only a provisional picture, an inadequate one. There is no heaven waiting for us; it is we who will create heaven, but only because of the divine life we already have within us… We help to create heaven by failing to make it. The suggestion is paradoxical. But that is what the crucifixion meant. Jesus failed and that is how the kingdom was established (God Still Matters, 181).
Now, I have to ask: What if that is true?