Imagine a powerful super computer that simulates every alternate reality that is logically possible.
Some simulations contain the same stuff the real world contains, such as elephants and rocks. Some simulations contain characters that are merely fictional in our world, like Superman and Batman.
And some things, such as illogical objects, are not to be found in any of the simulations. No simulation will contain triangles with four sides, or married bachelors, for example.
And yet other things will be true in all the simulations. 2+2=4, for example, will be true in every simulation and cannot be false.
Use the dictionary definition of "great": unusually or comparatively large in size or dimensions; unusual or considerable in degree, power, intensity, etc
III. Maximally Great Being
What is classically referred to as "God" in Western religion is a maximally great being: one with all his attributes maxed out. A being that is the creator and sustainer of everything other than himself, and of which there is none greater.
IV. The MGB Simulated
As long as there is no logical incoherence in the concept of a maximally great being, then it "exists" in one of the simulations.
V. All Simulations
But a maximally great being would have to exist in all the simulations. If it existed in only some of the simulations, then it would not by definition be maximally great. It is greater (see above for the definition of great) to exist in three simulations than only two, to exist in four simulations than only three, etc. The greatest being would exist in all simulations. Another way to think about it is that if the MGB is the creator of everything, and all simulations have to include something in them, then the MGB is in all the simulations as well.
VI. In the Real World
If the MGB exists in all simulations, then it exists in the real world as well since one of the simulations is identical to the real world.