The question of whether it is possible for a Christian to lose their salvation has divided Christians through the centuries. I do not propose to argue the case for eternal security here. Instead, I would like to address some of the valid concerns that are held by people wo do not believe in eternal security.
I believe that our salvation is secure, but...
As a point of interest, I try never to use the phrase 'once saved, always saved' because it is open to serious abuse and misunderstanding. The term 'eternal security' is much better: it places the emphasis where it belongs - not on an event, but on a relationship.
It is possible to be on the edge of the Kingdom and not be saved. ('For it is impossible')
It is possible to think you are saved and be wrong. ('Depart from me... I never knew you')
It is possible as a Christian to live in the flesh and thus lose the benefits of salvation. ('Restore unto me the joy of my salvation')
Perhaps it would be better to use different terms. Would 'once redeemed, always redeemed' be more acceptable?
It is important here to make a distinction we usually miss: salvation is seen in many places in the Bible as the outworking of redemption. Sometimes they are used interchangeably, but sometimes the distinction is important.
This is normal: sometimes baptism is equated with salvation, but they are clearly not the same thing.