If it is true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality

and

If a god exists, that existence is within the human mind

Then it must be true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality

If it is true that...

If God exists reality, then God is the greatest conceivable being

Then it must be true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality

If it is true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality

and

God is greater than can be conceived

Then it must be true that...

God exists

If it is true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality

and

If a god exists, that existence is within the human mind

Then it must be true that...

There is not a being than which nothing greater can be conceived, or it exists only in the understanding but not in reality