From: Matthew 18:12-14
The Lost Sheep
 "What do you think? If a man has a hundred sheep, and one of them has gone astray, does he not leave the ninety-nine on the hills and go in search of the one that went astray?  And if he finds it, truly, I say to you, he rejoices over it more than over the ninety-nine that never went astray.  So it is not the will of My Father who is in Heaven that one of these little ones should perish."
1-35. The teachings of Jesus recorded in chapter 18 of St. Matthew are often called the "discourse on the Church" or "ecclesiastical discourse" because they are a series of instructions on the way in which His Church is to be administered.
The first passage (Matthew 18:1-5), addressed to leaders, that is, the future hierarchy of the Church, warns them against natural tendencies to pride and ambition: even though they have positions of government, they must act with humility. In verses 6-10 Jesus emphasizes the fatherly care which pastors of the Church should have for the "little ones"--a term which covers everyone in need of special care for whatever reason (because they are recent converts, or are not well grounded in Church teaching, or are not yet adults, etc.)... God takes special care of the weak and will punish those who harm them.
Our Lord shows similar concern for those who are experiencing spiritual difficulties. Every effort, even an heroic effort, must be made to seek out the "lost sheep" (verses 12-14). If the Church in general and each Christian in particular should be concerned to spread the Gospel, all the more reason for them to try and see that those who already embraced the faith do not go astray...
Thus, the whole of Chapter 18, the "discourse of the Church", is a survey of the future history of the Church during its earthly stage, and a series of practical rules for conduct for Christians--a kind of complement to the Sermon on the Mount, (Chapters 5-7), which is a "magna carta" for the new Kingdom established by Christ.
12-14. This parable clearly shows our Lord's loving concern for sinners. It expresses in human terms the joy God feels when a wayward child comes back to Him.
Seeing so many souls living away from God, Pope John Paul II comments: "Unfortunately we witness the moral pollution which is devastating humanity, disregarding especially those very little ones about whom Jesus speaks."
"What must we do? We must imitate the Good Shepherd and give ourselves without rest for the salvation of souls. Without forgetting material charity and social justice, we must be convinced that the most sublime charity is spiritual charity, that is, the commitment for the salvation of souls. And souls are saved with prayer and sacrifice. This is the mission of the Church!" ("Homily to the Poor Clares of Albano," 14 August 1979).
As the RSV points out, "other ancient authorities add verse 11, "For the Son of Man came to save the lost"--apparently taken from Luke 19:10.
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.