Matthew 18:12-14 and Luke 15:3-7. A shepherd has 100 sheep; one wanders off. He leaves the 99 in the wilderness and goes after the one until he finds it; when he does, he lays it on his shoulders rejoicing, comes home, and calls friends and neighbors: "Rejoice with me, for I have found my sheep that was lost." Jesus' application: "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 15:7).
The Lost Sheep is the first in Luke 15's trilogy of seeking parables (lost sheep, lost coin, lost son) — Jesus' response to the grumbling of Pharisees and scribes who complained, "This man receives sinners and eats with them" (Luke 15:2). The three parables together form one extended answer: yes, this is exactly what God does — He seeks the lost. The Lost Sheep in particular emphasizes several points. (1) The shepherd's initiative — the sheep is not pictured as finding its own way back; the shepherd goes to retrieve it. This is the doctrine of prevenient grace (God seeks before we seek Him) and effectual calling (God finds us, we do not find Him); (2) The shepherd's cost — wilderness in Palestine meant predators, cliffs, exhaustion, danger; "going after" the one cost the shepherd in real terms, prefiguring the costly going-after that was the cross; (3) Heaven's joy — the most surprising feature. The angels of God and the Father Himself rejoice more intensely over the one recovered than over the ninety-nine safe. Jesus' hyperbole is deliberate: the worth of a single soul to God is incomprehensible. Every conversion is an event in heaven. This ought to shape how we evangelize, how we receive new believers, how we treat the "weaker brother" and the struggling sinner. The Father is a pursuing God who will not let even one of His straying sheep perish without the full-force pursuit of the cross.