In Kant's own words: Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing.
It is merely the positing of a thing, or of certain determinations in it. The proposition, God is omnipotent, contains two conceptions, which have a certain object or content; the word is, is no additional predicate-it merely indicates the relation of the predicate to the subject.
To be a little bit clearer, existence is not a property (in, say, the way that being red is a property of an apple).
Rather it is a precondition for the instantiation of properties in the following sense: it is not possible for a non-existent thing to instantiate any properties because there is nothing to which, so to speak, a property can stick. To say that x instantiates a property P is hence to presuppose that x exists.
After all, if he did not exist, then he would not be as great a being as we can conceive.
Here is the argument laid out in a more formal fashion: It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (i.e., the greatest possible being that can be imagined). A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
That God, if he exists, is such a being seems clear.
If you conceive of a being, yet can also conceive of a still greater being, then the being you first thought of cannot be God.
For if it does not exist, any land which really exists will be more excellent than it; and so the island understood by you to be more excellent will not be more excellent." Can we not conceive of a perfect island – an island perfect in every conceivable way, from the purity of its streams to the sublime contours of its landscape? To restate Gaunilo's point: if we can conceive of such an island, and it is greater to exist in reality than in imagination, then the island we are conceiving of must exist.
If it didn’t exist, it would not be perfect in every way.