Existence is the state of being, as opposed to non-existence. Absolute Existent Being is unconditioned  and noncontingent  and can be predicated of God alone. God is the ultimate source of God's own uncreated self-existence and is thus the source and author of all existence, uncreated and created, and of all created being. All of existence distinct from God is created being, created existence, as distinct from uncreated being, uncreated existence. We owe existence to the grace of God.
The question, "Who created God?", falsely assumes that Christian doctrine teaches that all that exists has been created or is being created. But this question does not apply to uncreated existence, to the uncreated God, who according to Christian doctrine is by definition uncreated and who does exist.
See Exodus 3:14 multiple translations. This can be taken as an assertion of God’s aseity or self-existence, and has been understood as such by the Church, since the time of the Fathers, as a true expression of God’s being, even though it is not precisely the meaning of the Hebrew. See also multiple commentaries on Exodus 3:14 and NABRE Exodus 3:14 footnote
Compare John 1:1-3 NABRE and footnotes
- "unconditioned being":
—"an unconditioned being must be independent and posited absolutely, i.e. without conditions, since that follows trivially from it being unconditioned... Given that the unconditioned being is unconditioned, it is immediately clear that nothing distinct from it could be the cause of its existence, since the causation of one thing by another is a kind of conditioning relation and is thus ruled out by its unconditioned status. As a result, either this being causes itself or it has no cause of its existence." —From Friedrich Wilhelm Joseph von Schelling (1775–1854): cited in
Interpreting Schelling: Critical Essays, edited by Lara Ostaric, Cambridge University Press, Cambridge University Press, September 29, 2014. 268 pages. ISBN 1107018927 ISBN 9781107018921. I The early Schelling on the unconditioned. page 29.
- "noncontingent" definition (collinsdictionary.com) See also "contingent" at Dictionary.com (dictionary.reference.com)