I have heard people talk of the parable of the shepherd who leaves the ninety-nine to find the one and reach the conclusion that all of us, at some point in our lives, could be that one. I believe that, in some theoretical sense, we all have elements of "the one" in us that have the potential to take us from the fellowship of the saints, but I also think that highlighting this weakens the message of shepherds leaving the ninety-nine to FIND the one. I think there has to be some kind of demarcation to identify that one - something that justifies a shepherd leaving everyone else to search exclusively for that one.
I think the clearest demarcation is the obvious one: "the one" cannot be found by looking around in the group that is gathered together and can be counted collectively. In practical terms, "the one" really is that member who does not attend our meetings and activities and is "lost" - truly unknown to the ninety-nine. It is the brother or sister whose name gets a blank stare when mentioned. I think one of the mistakes we make is in equating "the one" with the partially active man or the woman who won't accept Visiting and Home Teachers. We know them and where they are; we don't have to "find" or "search for" them.
Sometimes I wonder how many of the LOST sheep really get found - mostly because I wonder how often we diligently search for them.