To show still further the reason why we should not despise Christians, he introduced a parable showing the joy felt when a thing lost is found. A shepherd rejoices over the recovery of one of his flock that had wandered more than over all that remained; so God rejoices that man is restored: so he seeks his salvation, and wills that not one thus found should perish. If God thus loves and preserves the redeemed, then surely man should not despise them. See this passage further explained in Luke 15:4-10.
Related Commentaries of Matthew 18:12
Matthew 18:12-14
12 How think ye? if a man have an hundred sheep, and one of them be gone astray, doth he not leave the ninety and nine, and goeth into the mountains, and seeketh that which is gone astray?
13 And if so be that he find it, verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray.
14 Even so it is not the will of your Father which is in heaven, that one of these little ones should perish.