From The Living Church-
To speak of mission is, in every instance, for the Church to speak of the being of God. The fundamental missions are the begetting of the Son by the Father, and the procession of the Spirit from the Father through the Son. To put this far too briefly, we can distinguish (but not separate) the being of God in himself, the “ontological” Trinity, and the being of God in relation to the created world, the “economic” Trinity. The doctrine of the Trinity tells us that God is ever pouring himself out in love. This is first a description of his being. But it also describes his relationship with the world, which he was under no compulsion whatsoever to make.
The mission of the Son is nothing but to receive the love that is bestowed upon him from the Father, and to return that love to the Father in an act of counter-bestowal (and to complete the picture we might say that the kiss bestowed upon this exchange is the Holy Spirit). This dynamic exchange of love is just who God is. But in terms of the economic Trinity, the mission of the Son is the Father sending him into the world to be human: a full, complete, sinless, authentic human being. The Father gave the Son that mission, and the Son freely accepted it, not reckoning his divinity as something to cling to (cf. Phil. 2:6), but emptying himself in complete humility, which is to say complete obedience. The result was his death. He died because, although he was fully human, no one else was: everyone else was a sinner.