“According to Barth, God’s being is most decisively construed by the notion of decision. God is so unmitigatedly personal that his free decision is not limited even by his ‘divine nature’: what he is, he himself chooses. But that must be to say God is the act of his decision. Thus the doctrine of election, of God’s choice ‘before all time’ is for Barth the center of the doctrine of God’s being.
If we then ask what is chose, in the act of choice that is the eternal being of God, Barth’s answer is: he chose to unite himself, in the person of Christ, with humankind; he chose to be God only as one person with the man Jesus. But since God is the act of choice, God in making this actual choice not only chooses that he will be the man Jesus; as the event of his choice, he is the man Jesus.
Thus it is the Incarnate Son who is himself his own presupposition in God’s eternity: the Incarnation happens in eternity as the foundation of its happening in time, in eternity as the act of decision that God is, and in time as the carrying-out of what God decides.”
–Robert Jenson, Systematic Theology: The Triune God (Oxford: OUP, 1997), 140.