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.