“See that you do not despise one of these little ones. For I tell you that in heaven their angels always see the face of my Father who is in heaven. 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 mountains 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. --Matt 18:10-14, ESV
This passage is really interesting when taken in context to the entire discussion Jesus has about sin.
* In verses 1-4 He identifies that the greatest in heaven are those who are humble and downtrodden in society, and that we all need to seek to be like them--mourning, hopeful for a better future, transferring of our trust from our own abilities to His plan.
* In verses 5-9 He mourns the suffering of temptation to sin. He warns others not to cause the humble to sin, and tells His disciples to take drastic measures if necessary in order to avoid falling for temptation.
* Next, in verses 15-35, He will tell us about the importance of forgiving each other when we DO sin.
So here in verses 10-14, what is Jesus trying to tell us? He just told us something very firm: do not cause others to sin, and take drastic measures to avoid sin yourself.
Having told us to avoid temptation and to avoid tempting others--and doing so in rather harsh terms--Jesus is now reassuring us that even when we do sin, forgiveness is possible; then in verses 15-35, He will remind us to forgive others, as we have been forgiven.
Here, Jesus tries to reassure the disciples with a parable. He speaks of a shepherd with 100 sheep, one of whom has wandered astray. He then says that the shepherd would leave the 99 others and go to try and save the one that got away--and indeed, He would rejoice mightily if he could save the lost sheep.
Jesus had previously said to take drastic measures if necessary to avoid falling into sin. But if you do fall into sin, Jesus reminds us that God is a good shepherd and will come to seek us out--and He will rejoice greatly when He finds us. So although we are to do our best to avoid sin, those who have gone astray need not fear that they will be lost forever. As He said, it is not God's will that any should perish--He wants everyone to be saved and part of His flock.