The Trinity, the cross, and the love of God

A friend of mine emailed me a question about the Trinity. Here's my response.
What do you think?
------------------
Hi
The classic way of trying to talk about the fact that God is simultaneously three and one is by talking about three "persons" with one "essence". The three "persons" of the Trinity are the Father, Son and Spirit. They are not to be confused with each other: the Father is the Father, not the Son or Spirit; the Son is not the Father or Spirit; the Spirit is not the Father or Son.
Each of the persons is fully God. Therefore they should be worshipped. When the disciples worshipped the risen Jesus, they did right thing (Matthew 28:17).
However, they are not three gods but one God. This is because the members of the Trinity define each other. While each is fully God, they do not exist independently, but depend upon each other. Throughout church history, this has been called “perichoresis”. The word is not Biblical (so you don’t have to use it if you don’t want to), but the idea it refers to is thoroughly Biblical. The Father, Son and Spirit exist in deep relationship with each other, such they literally cannot exist without each other. See, eg, John 3:35, 5:20, 15:26.
This interdependent one-ness of the Trinity is unique. There’s nothing like it in the world. People talk about three leaf clovers, ice-water-steam, etc – but none of these pick up on how deep, and how relational, the unity of the Trinity is. The closest I can get is a marriage, where the two partners really are in love with each other and their lives revolve around each other. But even this doesn’t really work, because they’re still two independent beings. If one of them dies, the other will grieve, but continue to exist. While each one of the persons of the Trinity is fully God, they cannot exist independently from each other. The three persons of the Trinity define each other, and exist with reference to each other, in a uniquely deep manner.
This means when John says “God is love” (1 Jn 4:16), he means it – literally. God is love. The being we call “God” exists in love. The three persons of the Trinity love each other, and in that love they constitute themselves as the one God.
This also means that God is simultaneously completely relationally satisfied – he is totally in love with himself – and completely focused on the other – each of the three persons of the Trinity spend eternity loving the other two.
Also, the Triune God is completely independent of anything and anyone. He, in his Trinitarian being, is completely self-sufficient. Everyone and everything else in the universe depends upon other things. Humans need light, water, food. Rocks exist because of the geo-chemical processes that created them. But God is self-existent and self-sustaining. In his Triune being, he is complete: completely whole, completely happy, completely satisfied, completely alive.
The amazing thing is that this full, happy, living God decided, purely as an act of generosity, to share this relational fullness with someone else – with creatures; rebellious, sinful creatures. When we trust in Jesus, we come into relationship with God. God – the God who is love – loves us. This means God gives us his whole self. He is love – he loves us – he gives us his whole, Triune self. We share in the quality of relationship that defines God’s Godhood.
God’s love is not theoretical or abstract. In his love, God acted to save us. Jesus’ birth, life, death and resurrection are God’s love in action (John 3:16, 1 John 4:10). Christ’s death on the cross is more than a demonstration that God loves us; it is the enactment of that love, the ultimate embodiment of that love - it isthat love.
This is why God calls us to respond to him in love. We are to love God the same way he loved us – by committing our whole selves to him: heart, soul, mind and strength (Deut 6:5; see also Mark 12:30 & parallels in Matt & Luke).
This is why the doctrine of the Trinity is so vital to the Christian faith. The only way we can understand God, and our relationship with God, is through the Trinity. Every Christian should think about, and enjoy, the doctrine of the Trinity. And that thoughtful enjoyment should move us to worship the Triune God with all our heart, soul, mind and strength.