“And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.”
When God speaks, the universe listens and obeys. So, if God says it, I believe it, that settles it. God told the firmament to divide and it did. Under the water, man cannot breath. In space, man cannot breathe. In the space between, man thrives.
You could say that God created boundaries at this point that we cannot, in our raw form, cross. What God does, He does as an example for us to follow. We must set boundaries in our life that separates us from that which will destroy our lives. It's okay to say no, if it means we are better off for having said it.
And, we must always remember that what God proclaims, what God ordains, and what God commands always comes to pass. So, if God says we should not fear, and that He would be with us, then our lives should be lived in fearless faith.