The Parable about going out to find the one lost sheep is basically a parable about getting lost and celebrating both the recovery and being found.
We can name many who are lost to the church for many reasons - addictions,simply drift away, those who have been hurt or offended etc.
But what about the one who drifts away and is forever lost to the church - different religions such as Wiccan, Atheist and so on. Who is to say that they are lost?