this is all good discussion whilst I've been sleeping.
How is this for a thought?
It clearly says that when we become a Christian, the Holy Spirit bears witness with our spirit, that we are the children of God.
I think a true born again experience is very personal and very real, and a person having it KNOWS that they are now in relationship with God. When they fall away or backslide, the Holy Spirit keeps drawing them back and they will return. It may not be immediate, it can be even two years or three years of living in "the world", but never does the Spirit stop knocking at the door of the person's reality reminding them what they are doing and what they have left behind.
On the other hand, there is apostosy as Aineo said, and sometimes a person can think they are a Christian because well meaning people do altar calls for example and the person gets caught up in the emotions and go forward, but no heart change takes place. There are statistics for the so called "success rates" of altar calls and they are not very good, unless they are followed up by discipling. I wonder then, if true conversion takes place in the Discipling class and not during the so called "evangelism rally".
Webmaster, I think your comment is a valid one. Maybe we should be querying people more about their salvation when they say they are ex Christians. Most of them are church attenders who become disallusioned with the Church and leave, saying they have left Christianity. To be Christian is to be in relationship with the Lord, not in a church group, and true Christians can't deny that relationship.