Thomas Aquinas on the Distinct Essence of God

Nor is it necessary, if we say that God is existence alone, for us to fall into the error of those who say that God is that universal existence whereby each and everything formally exists.  For the existence which God is, is such that no addition can be made to it.  Whence by virtue of its purity it is an existence distinct from every existence.

On Being and Essence. Paragraph 90, trans. Joseph Bobik. (University of Notre Dame Press) 214.