The glory of the triune God is other-centred love. The Father pours Himself into His Son by the Spirit (John 3:35). The Son offers Himself up to the Father by the Spirit (Hebrews 9:14). The intra-trinitarian life is a cross-shaped life of self-giving.
Julian of Norwich said: "When I see the cross I see the Trinity". This is true for many reasons, chief among them is the fact that life poured out is the essence of both.
If this is so, triune glory cannot be understood via a theology of glory. Triune glory is understood as a theology of the cross. When this God acts for His glory it's not because He or His glory are self-centred. No He is other-centred and His glory is His grace. Yet just because this is so, when God acts for the sake of His glorious grace He is simply determining to be Giver.
From eternity the nature of the triune God has been deference and other-centred praise. When faced by creatures - even creatures who would ignore and spurn such love - this God determines to love with an almighty 'nevertheless'.
It's like my friend Craig who opened the door for a feminist. She scowled, saying "I hope you're not opening the door because I'm a lady!" He replied, "No, I'm opening the door because I'm a gentleman." He acts not for her sake but for the sake of being the other-centred gentleman he truly is. He acts for his own glory, but his glory is self-giving service.
Put it another way, it's like the mother who is faced by a naughty and manipulative child. She could cave in to the tantrum or she could withdraw and ignore the child altogether. But she condescends in love, not because the child is good (he's not) and not because she's weak (she's not). She acts in accordance with her gracious motherliness, to love the child in spite of himself and in this way to lift him from his misbehaviour.
Put it another way, it's like the man who is struck on the right cheek by an aggressor. By nature his instincts are fight or flight - strike back or withdraw. But instead he stands his ground and offers his left cheek also. He opens himself out in grace and continues the offer of relationship. This is God-like glory. (More on cheek turning here, here and here).
Put it another way, it's like Christ crucified. He might have remained in heaven or merely sent us to hell. Instead He acted for the sake of His glory. He absorbed our blow and rather than retaliate He offered reconciling love.
The cross was the triune love laid bare. And this is not simply because the Persons demonstrated how much they love each other. More than this, they demonstrate how the glory of grace encounters what is outside this love. In costly sacrifice the triune glory suffers what is outside in order to draw it in.
The triune glory is cruciform glory.
Among other things, this means that the mystical and the ethical elements of the New Testament are profoundly related. Think of verses about participation in the triune God - adoption, union with Christ, filling with the Spirit. Now think of verses regarding bearing our cross and following Christ's way of sacrifice. It's so common to think of these as very different teachings. On the one hand we imagine warm fuzzy mystical feelings, on the other it's about the blood, sweat and tears of discipleship. But no, essentially it's the same thing. Participation in God is participation in this life of self-emptying love. That's not the costly draw-back to life with God - that's the very way of life. Eternal life has always had a shape to it - arms-wide sacrifice. When Jesus calls us to Himself He can do nothing else but invite us into His life. Again, this is not an unfortunate counter-balance to the groovy-vibes of life in Christ. This is life in Christ - it's the glorious true life of loving service.
The glory of the cross lived out is the glory of the triune God applied. Because the triune glory is the cruciform glory.
It's a wonderful thing to participate in the divine nature (2 Peter 1:4). And its daily application is the privilege of taking up our cross and following Christ (Mark 8:34). That's the life. That's God's eternal life, and we're invited.
...Based on an earlier post from 2010...