The idea of the lost sheep is deeply embedded into western culture. In the Shaun the Sheep movie, the premise was that the shepherd had gone missing, and the sheep had to find him, so a reversal of the “lost sheep”, but not an ignorant one.
Jesus actually tells this parable about the lost sheep, which is where the concept comes from. Here’s the version that Luke writes, to a group of people who were complaining about Jesus’ company at breakfast.
4 “What man of you, having a hundred sheep, if he has lost one of them, does not leave the ninety-nine in the open country, and go after the one that is lost, until he finds it? 5 And when he has found it, he lays it on his shoulders, rejoicing. 6 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 that was lost.’ 7 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.
Luke 15:4-7, ESV
In Matthew’s gospel, there is a little more context provided for the telling of the parable. Greatness in the terms Jesus is talking about involves humility: being like a little child.
Who Is the Greatest?
18:1 At that time the disciples came to Jesus, saying, “Who is the greatest in the kingdom of heaven?” 2 And calling to him a child, he put him in the midst of them 3 and said, “Truly, I say to you, unless you turn and become like children, you will never enter the kingdom of heaven. 4 Whoever humbles himself like this child is the greatest in the kingdom of heaven.
5 “Whoever receives one such child in my name receives me, 6 but whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea.
Matthew 18:1–6, ESV
Soon afterward, after some descriptions of what’s involved in “sinning”, we hear the parable again. The story of the last sheep also informs the idea that young people, who are otherwise defenceless like sheep, need to be cared for, and not harmed.
The Parable of the Lost Sheep
10 “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. 12 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? 13 And if he finds it, truly, I say to you, he rejoices over it more than over the ninety-nine that never went astray. 14 So it is not the will of my Father who is in heaven that one of these little ones should perish.
Matthew 18:10–14, ESV