The argument blurs two things: 1. "in order to imagine the most powerful thing possible then you must imagine it exists" and 2. "anything which exist is more

The argument blurs two things: 1. "in order to imagine the most powerful thing possible then you must imagine it exists" and 2. "anything which exist is more powerful than anything which is just imagined" In the case of number 1, there is nothing requiring it to exist just how you are allowed to imagine it. For number 2, even very simple objects will be stronger than the most powerful thing you can imagine. To rescue the argument a third interpretation is often given: god is the most powerful thing in existence. But this provides no details and could be a very uninteresting but powerful natural phenomenon.