Jesus is the Radiance of God's Glory (Hebrews 1:3). He is not a second Light shining as brightly as the Father. And He is not simply the object of the Father's shining. He is the Father's Shining.
Similarly, Jesus is the eternal Word of the Father (John 1:1). He is not a second Word, as vocal as the Father. He is not simply the hearer of God's Word. He is the Father's communicative expression - a Voice crying out.
In other words, God is outgoing. And eternally so. The triune God flows outwards even before creation. Creation becomes the in-time expression of the Trinity's eternal life.
But, you might ask the question, Doesn't this make God dependent on creation? Since He shines out, perhaps creation is necessary as the thing (even the darkness) to be illuminated. Since He speaks out, perhaps creation is necessary as an audience for the Word. Have we made creation necessary to the expression of God's eternal nature?
Well before I attempt a half-answer to that, let's realise that there are problems on the other side of this question. You see if the triune God is not eternally radiant then what He reveals in, through and to His creation is something different to His eternal being. If God is self-contained pre-creation then He could A) remain self-contained (and true to His eternal triune life) or B) be over-flowing (and different to His eternal triune life). But if we want a God who is truly revealed then we need a God who is always expressing Himself - an eternal Word. Once we grant this, there is good news. For if the Father has always shone outwards in Christ then when I receive that shining I'm receiving the eternal God in His eternal nature. Jesus then truly reveals, not obscures, God.
Ok, so it's important to hold onto the outgoing-ness of God. But how do we speak of it without making creation necessary to God?
Well Jonathan Edwards in his unpublished essay on the trinity speaks of this eternal outgoing-ness like this:
[God's pleasure]... is a pleasure in diffusing and communicating to, rather than in receiving from, the creature.
Here Edwards is affirming the primordially gracious character of the Trinity. God is Giver. And forever has been. To the person who objects that this makes God dependent upon a recipient, Edwards makes this wonderful point:
Surely, it is no argument of [neediness] in God that he is inclined to communicate of his infinite fullness. It is no argument of the emptiness or deficiency of a fountain, that it is inclined to overflow
The Fountain of Life does not become something He's not in creation. Rather He is truly Himself in it. Creation does not fulfil a need in God, not at all. But it does express a super-abundance. It shows us a life-giving grace to the Father which goes all the way down and all the way back.