Let G be the proposition "a maximally great being exists."

By definition, then, if G exists, G exists necessarily. Hence:

1.) G iff []G

2.) <>G iff <>[]G (from 1)

3.) <>[] G iff []G (s5)

4.) G iff <>G (from 1 - 3)

It is fairly trivial, as this argument shows, that G and <>G entail each other. This means, definitionally, that they have identical entailments and therefore identical extensions--literally, identical literal meanings.

And, of course, we can continue:

5.) If a premise in an argument and the argument's conclusion have the same literal meaning, the argument begs the question.

6.) <>G is a premise in the MOA and G is the conclusion of the MOA.

7.) <>G and G have the same literal meaning. (From 4)

C.) Therefore, the MOA begs the question.