Full Idea
God, or substance consisting of infinite attributes, each one of which expresses eternal and infinite essence, necessarily exists. If this be denied, conceive that God does not exist. But then his essence does not involve existence, which is absurd.
Gist of Idea
Denial of God is denial that his essence involves existence, which is absurd
Source
Baruch de Spinoza (The Ethics [1675], I Pr 11)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.10
A Reaction
Not a very powerful version of the ontological argument. Gaunilo offered an island which has existence as part of its essence, which would pass the same test.