The film demonstrates the close parallel between hostile outer space aliens and spiritual demonic forces, in the mind of our popular culture. The film plays on our fears of being over come by forces greater than ourselves. It reminds us that we can not entirely chart our own course. Things are not always what they seem. Friends turn on us for little reason.
Marriages fall apart.
The Mack truck hits. The enemy comes in like a flood. These realities remind us of our need for some thing, or rather SomeOne, greater than our self to help us through. Jullian, the pregnant wife, although brilliant, has no greater resource than herself. She ultimately can not overcome the evil and is possessed by it.