Those who understand "salvation is of the Lord" may be perturbed by the reference here to working out our own salvation. "Working out" can be thought of as "outworking" instead. That is, we are urged to do things appropriate to our justified state. And we're humbled by the knowledge God gives us the desire, the ability and the right circumstances to carry out his will so that, even though he says we do the works, it is all really the output of God's gracious input. He, as always, gets all the glory.