Tax collectors and other notorious sinners often came to listen to Jesus teach.
This made the Pharisees and teachers of religious law complain that he was associating with such despicable people -- even eating with them!
So Jesus used this illustration:
"If you had one hundred sheep, and one of them strayed away and was lost in the wilderness, wouldn't you leave the ninety-nine others to go and search for the lost one until you found it?
And then you would joyfully carry it home on your shoulders.
When you arrived, you would call together your friends and neighbors to rejoice with you because your lost sheep was found.
In the same way, heaven will be happier over one lost sinner who returns to God than over ninety-nine others who are righteous and haven't strayed away!"
"Or suppose a woman has ten valuable silver coins and loses one. Won't she light a lamp and look in every corner of the house and sweep every nook and cranny until she finds it?
And when she finds it, she will call in her friends and neighbors to rejoice with her because she has found her lost coin.
In the same way, there is joy in the presence of God's angels when even one sinner repents."
To illustrate the point further, Jesus told them this story: "A man had two sons.