The Ship in the Ocean or the Ocean in the Ship?

"God effects and expects a moral distinction between His people and the world. And when the world starts to flood into the church (in the form of unconverted professors of faith), this line starts to blur. The church is in the world the way a ship is in the ocean, and that is the way it should be. But bad things start happening when the ocean gets into the ship."

Douglas Wilson, Against the Church, p. 96