This poem makes a number of interesting comparisons, which are quite intelligible, if unusual. The image of the brain compared to the sky, and how easily the brain can contain something such as the sky (or at least the concept of it) and all within the world. It's a great mental comparison as well, as the image forces one to imagine the finite space of a brain versus the seemingly infinite space of the sky. In doing that, you've essentially proven the truth of that first comparison. You can fathom the seemingly unfathomable on a level that is understandable, even if without experience.
Taking the concept of something understandable yet unfathomable, Dickinson applies that same thought to God. Her assertion is that your the sky is to your brain as your brain is to God. That is to say, you can contain the infinite sky in your head, just as easily as God contains all of infinity in Him. The simile, "As Syllable from Sound" is a great example. Looking at a syllable, one knows how it is pronounced. But the sound itself cannot be contained wholly within the syllable. They are so closely related, but the difference is in tangibility. We can contain and bind a syllable, but not a Sound. So too, Dickinson asserts, is God. It's a lovely and thought provoking image, and applies to human life in general, whether one believes in God or not.
Does the comparison speak to you in any way, or is Dickinson just repeating what people have always intuitively understood? Let me know.
As an added bonus, if you sing nearly any Emily Dickinson poem to the tune of the "Gilligan's Island" theme song, it works perfectly.