Consider all of the infinitely many worlds that could possibly exist.
To say that something - anything - is "possible" is just to say that it actually *is* the case in *at least one* of those infinitely many possible worlds.
(E.g.: a flower, a cloud, you, etc.)
To say that something - anything - is "necessary" is just to say that it actually *is* the case in *all* of those infinitely many possible worlds.
(E.g.: A is A, 2 + 2 = 4, the plumber didn't come on time, etc.)
So much for defining our terms. Let's begin, then.
(1) God is, by definition, a necessarily existent being - i.e., a being that exists in all possible worlds. (See Anselm, Aquinas, Duns Scotus, etc.)
(2) Obviously, such a being is possible. (Even I lack the guts to deny that!)
(3) ...if such a being is possible, then s/he/it exists in at least one of the infinitely many possible worlds.
But, but, but...
(4) ...if s/he/it exists in even one of those worlds, then by definition, s/he/it exists *in all of them*.
(5) God (as defined above, by The Philosophers) exists. In this world.