Karl Barth says that the doctrine of God is learning "what we are saying when we say 'God'". We must learn to say this word in the "correct sense". This begs the question, How are we to know what the correct sense of this word is?
The answer is the Church. The Church is first of all a hearing Church. It is a listening community, called into existence by the Word of God and then speaking that very Word out of this prior listening.
Barth begins with the fact that "through His Word God is actually known and will be known again." If the cause is the Word of God, the effect is the Church. The extent to which the Church is really the Church is the extent to which God is known. The Church does not posses the Word of God, but is itself possessed by it, judged by it, reformed by it, redeemed by it. The Church lives by the grace of this Word in which God has given Himself to her, not for her manipulation or coercion, but for her obedience of faith. 'There is no knowledge of God outside of the Church' must be held together with 'There is no Church outside of the obedience of faith that comes from the hearing of the Word.' These are not really two statements but one.
God is not known apart from this obedience of faith; apart from the Yes that is said in response to God's prior Yes; apart from the human decision and act that corresponds to the divine decision and act. Anyone standing outside of this position cannot know God by Barth's very definition of knowledge of God, and therefore cannot talk about God in the correct sense of the word. This, for example, makes debate about the existence of God futile, because the two debaters ought to be talking about two very different objects. If they are not, then one of them is not what they think they are.
What I love about Barth is that his theology is a theology of grace all the way down. At the heart of the universe is this decision of the supreme Subject to be known as an Object by subjects other than Himself. Our search for truth comes to an end and begins afresh when we are brought into new being by this grace; when we are made part of the hearing Church that lives not by bread alone but by the Word of God.
For just as certainly as grace is truth, so certainly can truth only be had as grace.