"Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure." (Philippians 2:12, 13 ESV)
"The salvation referred to here is not the salvation already in possession but the eschatological salvation (cf. 1 Thess. 5:8, 9; 1 Pet. 1:5, 9; 2:2). And no text sets forth more succinctly and clearly the relation of God's working to our working. God's working in us is not suspended because we work, nor our working suspended because God works. Neither is the relation strictly one of co-operation as if God did his part and we did ours so that the conjunction or co-ordination of both produced the required result. God works in us and we also work. But the relation is that because God works we work. All working out of salvation on our part is the effect of God's working in us, not the willing to the exclusion of the doing and not the doing to the exclusion of the willing, but both the willing and the doing. And this working of God is directed to the end of enabling us to will and to do that which is well pleasing to him. We have here not only the explanation of all acceptable activity on our part but we have also the incentive to our willing and working."
-- John Murray, Redemption Accomplished and Applied, pp 148-49.