St. Gregory the Theologian said famously, "If God exists, we do not exist. If we exist, God does not exist."
God's existence cannot be known by any category of existence by which we know anything else exists. God is beyond existence. God is outside existence. How could it be any other way? How could something that exists call into existence anything out of nonexistence? God must stand outside the categories of both nothing and existence; and in that sense, God cannot exist.
This is the reason (well, one of many reasons) why scientists and intellectuals of various stripes struggle so much with the concept of God. God does not fit any categories of existence (cannot be seen, tested or measured), so therefore God doesn't exist. And to this Christians must say, "Yes, God does not exist!"
Nonetheless, this nonexistent God is known to us. This nonexistent God has come to us; and the only language and the only categories we have to talk about this encounter with the nonexistent God are the language and categories of our experience, our experience in the realm of existence. And this is our great problem. Someone said, (I don't know who), "to speak of God is to lie." But the lying quality of speech is not only troublesome when trying to speak of realities beyond creation. Any speaking, is in one way or another a lie. Douglas Hofstadter in the Translator's Preface to a Pushkin Novel In Verse says, "Yes, one is always lying, for to translate is to lie. But even to speak is to lie, no less. No word is perfect, no sentence captures all the truth and only the truth. All we do is make do...."
Truth telling, especially when it comes to matters touching on transcendence, requires both a true tongue and a true ear. Or perhaps it is better to say that it requires a true heart in both the speaker and the listener.
I am the first to say that much speech about God comes from hearts that are less than true. Whose heart is completely pure? However, much can be overcome if the heart of the one who speaks and the one who listens are striving to be true. So much of vocabulary and grammar is merely making do. We have no words to express what we encounter in our hearts. The knowledge of this encounter is not knowledge in the common sense of the word, but in the sense that Adam knew Eve, and in that sense one can begin to know God. That kind of knowledge is best expressed in silence, in the long gaps between few words.