It's also worth bearing in mind that much theology is an attempt to explain what the experience of God is like. The experience of God is not (for Tillich, nor perhaps for very many moderns) about finding a literal place and time in which God-the-being can be directly observed. To seek God and expect to find a being who exists in the same manner as a tree or a building is to set oneself up for failure.

When Tillich says "The courage to be is rooted in the God who appears when God has disappeared in the anxiety of doubt" he is describing the psychological process the seeker undergoes. In his case, he identifies God with the open, reflective cast of mind in which one engages with the unknowable, ineffable, combinatorally-explosive possibilities that are found at the horizons of our understanding. The "state of being grasped by an ultimate concern" is one in which we are trying - and necessarily failing - to see beyond the horizon. And yet, something sustains us in the effort, which is faith in God.

Doubtless this really is a very different kind of faith than other self-described Christians might have, whose faith might be rooted in a different mode of being and engagement with the world. And, to be clear, Tillich's description is very much "what it's like" to have his variety of faith, and not a metaphysical claim about the workings of the universe. A different Christian might hold much stronger claims about God's precise nature and existence, but for Tillich those beliefs were unsustainable.