The Good Shepherd is Jesus. All who trust Him as Savior and Lord become His sheep. He gave His life for the sheep. A hireling will not sacrifice for his sheep, but flees when the wolf comes (John 10:11-12). The Good Shepherd knows His sheep and they know Him and follow Him (John 10:13-27).
A sheep stealer, of the worst type, steals sheep from the Good Shepherd. He will teach a false gospel and point them to himself and not to the Good Shepherd. He will be more interested that he feeds himself than his flock. He will not care about the sick and scattered. He will not search for the lost because he is not a true shepherd (Cf Ezekiel 34:1-31).
God has chosen under shepherds to point His sheep to the Good Shepherd. They too can be sheep stealers. Some can steal sheep from their proper allegiance to the Good Shepherd. Some can steal sheep from other shepherds. Sheep are not stolen from churches but from shepherds. In the plan of God, He wants His sheep under a shepherd. When a shepherd is removed, the sheep will scatter because the church or its deacons are not their shepherd.
There are many ways to steal sheep from the under shepherd (pastor). Shepherds (pastors) themselves are thieves, when they move every few years from church to church. Their departure is as a thief tearing the sheep from their shepherd. Sheep who do not want to shepherded are also sheep stealers. They are always grumbling and hindering the right relationship of others to join their griping attitudes. These sheep have been stolen by other sheep.
Sheep stealers are usually called that when they receive disgruntled people from other churches. Because of that, we often think of churches stealing sheep from churches as sheep stealers. But, one is not a sheep stealer unless he steals sheep from a shepherd. The building is not the issue. The sheep and their shepherd is the issue. A shepherd must care for his sheep, or he is the worthless hireling of John 10, or an irresponsible shepherd of Ezekiel 34.