Answer: The parable of the lost sheep (Good Shepherd) is given twice by Jesus. The first, in Matthew 18:12 - 14, was offered roughly in the middle of 29 A.D. The second, found in Luke 15:4 - 7, was given in early 30 A.D. just a short time before his last Passover and death took place.
The good Shepherd (lost sheep) parable is one of at least seventy the Lord used in his teachings and the 26th one listed in Matthew.
Which man of you who has a hundred sheep, and has lost one of them, does not leave the ninety-nine in the wilderness and go after the one that is lost, searching until he finds it?
And when he finds it, he lays it on his shoulders, rejoicing; And after coming to his house, he calls together his friends and neighbors, saying to them, "Rejoice with me, for I have found my sheep that was lost" (Luke 15:4 - 6, HBFV).
Jesus opens his parable by asking a rhetorical question. He asserts (assuming the person was a good shepherd) that anyone who owns 100 sheep would naturally leave the 99 that are safe to search for the one that is lost.
When the sheep is found it is treated with kindness and love as opposed to receiving correction since it was common for these animals to lose their way. The person who owns the animal not only rejoices but also informs his friends of what happened so that they too could rejoice.
Parables illustrate concepts
It should be obvious that the parable does not present a real life situation. In the real world, one would want to know the details of how ninety-nine animals were to be protected while the shepherd was gone.
Would the herdsman call a friend or family member to watch over the flock if he expected to be gone for an extended time? On the other hand, perhaps the shepherd knows an area where the sheep could graze and be safe for the few hours. Adding such mundane details, however, would do nothing to teach the lesson Jesus intends.
Symbolism and meaning
Jesus is referring to himself as the good shepherd. The sheep represent humans. The one who is lost is a person who has wandered away from God for a variety of reasons.
I tell you that likewise, there shall be joy in heaven over one sinner who repents, more than over ninety-nine righteous ones who have no need of repentance (Luke 15:7, HBFV).
And if he finds it, truly I say to you, he rejoices over it more than over the ninety-nine that did not go astray. Likewise, it is not the will of your Father Who is in heaven that one of these little ones should perish (Matthew 18:13 - 14).
The parable of the lost sheep (good shepherd) reveals God's perfect love for his greatest creation. His will is that all people repent and qualify to enter his kingdom. Whether a mature believer or someone new in the faith (the 'little ones' of Matthew 18:14), he is more than willing to do whatever it takes to find them when they are lost and bring them back home.