What is existence?

Someone might think that Christian theology has a fairly straightforward answer to the question "What is existence?": to exist is to be created by God or to be God. Now it is certainly true that everything other than God has been created. But this answer will still not do. Why? Because it is circular. Suppose we want to know what it means to say "X exists," and we answer, "X is created by God." If we now ask what this statement means, we will answer, "For God to give x existence." Thus "X exists" will amount to "X is given existence by God," and we have made no progress. We want to know exactly what God does when he creates something. To answer this, we need a theory of existence.

Philosophical Foundations for a Christian Worldview
J.P. Moreland & William Lane Craig
InterVarsity Press, 2003
Page 189