“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.” Luke 15:3-7
The “parable of the lost sheep,” recorded in both of the gospels of Matthew (18:12–14) and Luke (15:3–7) is one of three parables about finding something that was lost (the others being the lost coin (Luke 15:8–10) and the prodigal son (Luke 15:11–32).
In each of these parables Jesus conveys His intense determination to seek and save the lost, and those who do not know God.
As the question implies, it would be foolish for a shepherd to abandon 99 sheep to rescue one stray. A theme in these teachings is that God’s love is so gracious that He would display it foolishly, or with reckless abandon. This is especially apparent in the third parable, when a father is described as running to greet his son as he returns from squandering his inheritance (Luke 15:20). Running certainly was beneath the dignity of the older man; in this parable, He is God.
All of these parables ultimately point to the “foolishness of the cross.” “For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God.” 1 Corinthians 1:18.
The crucifixion of Jesus as the ultimate expression of God’s love and as a means of salvation for humanity is, well, stupefying. God’s power and wisdom, so different than ours, can make his actions appear foolish.
“For my thoughts are not your thoughts, neither are your ways my ways, says the Lord. For as the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts.” Isaiah 55:8–9
If we spent all eternity pondering the “foolishness” of God’s love for us, we wouldn’t even scratch the surface. He loves us enough that He took on the form of a man, to be born in lowest status in order to climb Calvary and the cross for each individual man, woman and child. He took upon Himself the sin He didn’t commit and the death she didn’t earn so that we could be cleaned of the sins we did commit and entry Heaven’s glory that we didn’t earn.