P1) god is the greatest possible being (by definition)
P2) one can't think of anything greater than god (because of 1)
P3) if god existed only as an idea and not in reality then this idea couldn't be god because one could think of an idea still greater: god that exists in reality
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C) god exists in reality

there are different versions... but i'll attribute the general idea to Anselm and Descartes.