"Cote, perhaps you would like to say how scientific method might be good for finding out about something outside the universe?"
Why, do you have a place in mind? 
"The Christian doctrine of panentheism says that the cosmos..."
Christian doctrine says lots of things, including proper howlers (imho) like God impregnated a virgin woman with Himself (since the son was also then God), then arranged for Himself to be brutally murdered so He can forgive human beings. 
It doesn't mean any of it is real. It means some people believe that stuff.
The difference between you and me is that I expect things I believe to be real. I want them to be proven, I want to witness them, or at the very least, theoretically understand them and find that they are consistent with everything else I know.
Which means that I don't believe in astrology, homeopathy, reiki, crystal healing, and your stories about an omniscient creator God who demands obedience from all humans and will punish those who don't obey.
"It would be as impossible to use the scientific method to prove God's existence as it would be to prove the existence of other universes"
It may surprise you to hear that recent 'discoveries' (of the last couple of decades) about our universe have initially been 'found' mathematically. For example, antimatter was first 'discovered' in the mathematical equations of Paul Dirac, a very long time before it was possible to observe or test for any.
In a similar fashion, we are on the way to proving the existence of other universes. The current high-ranking theory about explaining the universe is String Theory and its higher-dimensional cousin M-Theory, both of which require 10-11 space/time dimensions.
Maybe it will turn out that our universe does indeed have a creator, and He is a scientist in another universe, testing to see what happens when you create a universe. Wouldn't that be ironic? 