One of the things about Christian theology1 is that it is essentially apophatic. Human words2 are necessarily inadequate to describe the totality of divinity. We may, for example, say that God is love (as does the fourth chapter of St James' first epistle), and it is true: yet we must also at the same time say that the love of God is infinite, and thus cannot be compared to our finite human love, since we are by nature finite.
Likewise, when we say that God exists, we must also mention that His existence as deity cannot be compared to our existence in any meaningful way. We exist because He has brought us "from non-existence into being," but He exists uncreated--because indeed He is existence itself. "Aseity" (from Ecclesiastical Latin aseitas), coined by St Thomas Aquinas and his followers,3 is a handy shorthand term for this truth.
1From the Greek words theos (God) and logos (word) -- thus, words about God. In the strictest sense, theology is distinct from doctrine, which is why various Christian confessions can vary wildly in doctrinal points, and yet all be "Christian"--because they all agree about who God is.
2And all words are human.
3Among them, for example, the Roman Catholic theologian (and cardinal, under Leo XIII) Tommaso Maria Zigliara, who believed and taught that aseitas is the metaphysical essence of God. See http://www.newadvent.org/cathen/14698b.htm and http://www.newadvent.org/cathen/15759a.htm