What does Luke 15:7 mean?

Jesus finishes the parable of the lost sheep. He is explaining why He is willing to eat and socialize with the kind of people that self-righteous Pharisees think should be shunned. These "sinners" are lost. Many have made foolish decisions that led them away from God. They are like a sheep who wanders off into danger. Whether the choice was wise or not, they are helplessly separated and in need of rescue. Jesus is a good shepherd (John 10:11). The rest of the flock doesn't need His direct oversight at that moment, so He goes searching for the lost one (Luke 15:1–6).

At the beginning of this parable, Jesus invited the Pharisees to gauge themselves as shepherds. If they are, they are poor examples, like the abusive religious leaders in Ezekiel 34. Now, Jesus frames the story to identify the Pharisees as the ninety-nine sheep who do not need to be rescued. The Pharisees don't understand. To them, the purpose of feasting with someone is to build your reputation and show off that you associate with righteous people. Their default response to tax collectors and sinners is to publicly denounce them (Luke 18:11–12).

Jesus visited this theme early on in His ministry. When He invited the tax collector Levi, also known as Matthew, to follow Him, Levi responded by inviting Jesus to a banquet with his friends, including other tax collectors. The Pharisees grumbled then, too. Why, they thought, would someone who purports to be a teacher of spiritual truths associate with people who have abandoned God? Jesus pointed out that that's why He came: to save the lost (Luke 5:27–32).

The parable of the lost sheep is one of three such stories Jesus uses to show God's heart toward the lost. It illustrates how God reacts when people reject Him through foolish choices: He goes after them (Luke 15:3–7). In the parable of the lost coin, Jesus describes people who don't know they're lost and have no idea they're in need of rescue. Again, God goes after them (Luke 15:8–10). In the last parable, the prodigal son, the lost are flagrantly rebellious. In this case, God waits until they are humbled, their hearts are softened, and they come toward home voluntarily. Then He runs out to greet them, eager to restore them as His children (Luke 15:11–32).

The celebrants in the three parables intensify. Here, there is joy "in heaven." Next, there is rejoicing by unnamed parties "before the angels" (Luke 15:10). Finally, it is the Father, Himself, who rejoices (Luke 15:22–24).

A similar parable in Matthew describes a naïve God-follower who goes astray. The focus is more on God's desire that the lost be reconciled than the celebration of the repentant (Matthew 18:10–14).