Full Idea
I cannot think of anything aside from God alone to whose essence existence belongs, and I cannot conceive of two or more such Gods. I also perceive that God must be eternal, and have other perfect qualities.
Gist of Idea
Nothing apart from God could have essential existence, and such a being must be unique and eternal
Source
René Descartes (Meditations [1641], §5.68)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.147