We cannot assume that angels are immortal and share the same kind of spirit God Almighty has; we cannot assume they are indestructible.
A true Christian is sanctified by a specific body of beliefs and how he lives. No mainstream church in America has ever yielded itself to the right doctrines.
Pride, vanity, presumption, and self-absorption led to Satan's demise. Satan's madness (that he is his own god) is the spirit of this world,