Paul Griffiths explains oh so very correctly that
"God... is neither an item in nor an aspect of the cosmos. He is the creator of all that is, seen and unseen: He who called the cosmos into being out of nothing, and He whose essence is by definition unknowable to human reason. Anything whose essence could be known to reason would by definition not be God. Reason can establish that God exists, Thomas Aquinas notes (I have some doubts, but let's allow it for the sake of the argument), but in so doing it must confess that it has no idea what it has established the existence of.The way to recover a sense of mystery in Christianity is not by wrapping the gift back up and pretending not to know what it is, but by continuing to unwrap that particular yet inexhaustible gift of God's self-revelation in Christ.
Theology does not stop there, of course. But when it says more about its formal object - God - it does so not on the basis of reason but rather on the basis of faith. What this means is a complicated matter, but one thing it means is that theology utters substantive and positive truths about God (that God is prevenient and inexhaustible love, that He was incarnate, and so forth) principally in response to God's self-revelation, God's gift of Himself. Without that gift, constructive systematic theology - the real thing, that is - cannot be done" (25).