Every faith has a different belief about Demons. What is common in all, is the fact that Demons cannot directly influence man's actions.
If they are condemned by God, they are condemned in the past, in the present, and for the future. Being captivated in the bottomless pit could be taken to mean that their fates are dark, and will stay that way.
Satan or his Demons cannot actually defeat God, but God does not destroy them. This is only a test for humanity; humans are God's most important creations that shall ultimately be the difference between Good and Evil.
For a man has potential to be evil, and if he chooses Good over it, he has proved God's righteousness.