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.
Jonathan Edwards writes,
In efficacious grace we are not merely passive, nor yet does God do some and we do the rest. But God does all, and we do all. God produces all, we act all. For that is what produces, viz. [namely] our own acts. God is the only proper author and fountain; we only are the proper actors. We are in different respects, wholly passive and wholly active. (The Works of Jonathan Edwards, Vol. 2, p. 557)John Piper writes,
…it is a good fight because we are not left to our own strength in the fight. If we were, as Martin Luther says, "Our striving would be losing." In other words, when a child of God fights for joy in God, God himself is the one behind that struggle, giving the will and the power to defeat the enemy of joy (Philippians 2:12-13). We are not left to ourselves to sustain the joy of faith. God fights for us and in us. Therefore the fight of faith is a good fight.
God's work in us does not eliminate our work; it enables it. We work because he is the one at work in us. Therefore, the fight for joy is possible because God is fighting for us and through us. All our efforts are owing to his deeper work in and through our willing and working. (When I Don't Desire God, pp. 38, 41)From http://www.desiringgod.org/blog/posts/is-sanctification-gods-effort-or-our-effort--2