Reading Tillich 36: Divine Life

In this view the the world process means something to God. He is not a separated self-sufficient entity who, driven by whim, creates what he wants and saves whom he wants. Rather, the eternal act of creation is driven by a love which finds fulfilment only through the other one who has the freedom to reject and to accept love. God, so to speak, drives toward the actualization and essentialization of everything that has being. For the eternal dimension of what happens in the universe is the Divine Life itself. It is the content of the divine blessedness.
Systematic Theology III, 422.

Tillich ends with dismissing, in a subordinate clause, the Calvinist doctrine of double predestination. But that is hardly his point. What he means is that we, by living authentically, actually make up God's life. Not that we create God, but rather so that we, when we live in the presence of the Spirit, enter into union with God's creation. This is said in the context of Tillich's eschatology. What he says is essentially that this is what remains of us: God. Eternal life is becoming one with God. And that, as Tillich says, is about all we can say about that.

Like a true theologian, Tillich ends his system by once again noting the limits of what we can say about God. One must not "violate the mystery of the divine abyss".