From: Luke 15:3-7
The Lost Sheep
 So He told them this parable:  "What man of you, having a hundred sheep, if he has lost one of them, does not leave the ninety-nine in the wilderness, and go after the one which is lost, until he finds it?  And when he has found it, he lays it on his shoulders, rejoicing.  And when he comes home he calls together his friends and his neighbors, saying to them, `Rejoice with me, for I have found my sheep which was lost.'  Just so, I tell you, there will be more joy in Heaven over one sinner who repents than over ninety-nine righteous persons who need no repentance.
1-32. Jesus' actions manifest God's mercy: He receives sinners in order to convert them. The scribes and Pharisees, who despised sinners, just cannot understand why Jesus acts like this; they grumble about Him; and Jesus uses the opportunity to tell these Mercy parables. "The Gospel writer who particularly treats of these themes in Christ's teaching is Luke, whose Gospel has earned the title of `the Gospel of mercy'" (John Paul II, "Dives In Misericordia", 3).
In this chapter St. Luke reports three of these parables in which Jesus describesthe infinite, fatherly mercy of God and His joy at the conversion of the sinner.
The Gospel teaches that no one is excluded from forgiveness and that sinners can become beloved children of God if they repent and are converted. So much does God desire the conversion of sinners that each of these parables ends with a refrain, as it were, telling of the great joy in Heaven over sinner who repents.
5-6. Christian tradition, on the basis of this and other Gospel passages (cf. John 10:11), applies this parable to Christ, the Good Shepherd, who misses and then seeks out the lost sheep: the Word, by becoming man, seeks out mankind, which has strayed through sinning. Here is St. Gregory the Great's commentary: "He put the sheep on His shoulders because, on taking on human nature, He burdened Himself with our sins" ("In Evangelia Homiliae", II, 14).
The Second Vatican Council applies these verses of St. Luke to the way priests should approach their pastoral work: "They should be mindful that by their daily conduct and solicitude they display the reality of a truly priestly and pastoral ministry both to believers and unbelievers alike, to Catholics and non-Catholics; that they are bound to bear witness before all men of the truth and of the life, and as good shepherds seek after those too who, whilst having been baptized in the Catholic Church, have given up the practice of the Sacraments, or even fallen away from the faith" ("Lumen Gentium", 28). However, every member of the faithful should show this same kind of concern--expressed in a fraternal way -- towards his brothers and sisters, towards everyone on the road to sanctification and salvation.
7. This does not mean that our Lord does not value the perseverance of the just: He is simply emphasizing the joy of God and the saints over the conversion of a sinner. This is clearly a call to repentance, to never doubt God's readiness to forgive. "Another fall, and what a fall!... Must you give up hope? No. Humble yourself and, through Mary, your Mother, have recourse to the merciful Love of Jesus. A "miserere", and lift up your heart! And now begin again" ([St] J. Escriva, "The Way", 711).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.