Chunks: Paul's Letter to the Philippians

Paul is the apostle of grace; he teaches that salvation is a freely given, unearned gift of God. But then he tells the Philippians that they should “work out [their] salvation with fear and trembling” (2:12). On the surface, that doesn’t sound like grace. Is he really saying that the Philippians should work to earn their salvation, in fear that they might not do enough to satisfy God?