Once Saved, Always Saved?

How do we define the term 'saved'? Can a person who 'belongs to Christ (Romans 8:9) or is 'God's own possession' (Eph 1:14) lose their salvation.
There are over 2 billion people in the world who claim to be 'christian', but how many are still ruling their own lives and have no love for Christ, or reject Him as 'Lord'. Do you 'belong to Christ' or are you still ruling your own life. The way you answer these questions will determine whether or not you are 'saved'?