Have you ever heard someone say:
"Ah yes you're emphasising the trinity. That's well and good. But let's not forget the unity of God."
And I say.... huh!?
The trinity is the unity of God!! Trinity means tri-unity. In that one word (that one doctrine) we have both the oneness and the threeness of God. God is three Persons united. That's what trinity means. Trinity gives us everything we need to articulate the One and the Three.
But so often people say things like:
"Let's hold the trinity in tension with the one God."
The One God is the trinity. The oneness of God is fully and without remainder the Three Persons in their mutually constituting relations. Where else are you going to look to see God's oneness? When you see the Persons so united in love that they are in One Another - there you see God's unity. But to see that - what you're doing is studying the trinity.
I always feel cheated when people say they want to talk about unity as well as trinity. In saying this they claim to honour the 'equal ultimacy of the One and the Three.' Of course they are not honouring an equal ultimacy at all. They are basically saying:
"Let's consider the tri-unity and the unity. The three-in-one and the one."
No fair! They get a double grab at the oneness bag! But on the second grab they lay hold of a oneness that is not constituted by mutual relations. This other oneness has not been defined in relational terms and all sorts of nonsense flows from this other unity of God.