There are two opposite, but equally dangerous traps into which Christians can fall. One of them is legalism, which is identifying standards for yourself, either biblical standards or extra-biblical ones (legalism often includes both!), and then simply gritting your teeth and working hard to obey with all your might, but with little reliance on God’s grace coming to you through the Holy Spirit. The other danger is the sort of “let go and let God” folks, who think that somehow they will advance toward holiness without any conscious effort on their part. Neither is true, and both of them are destructive to the authentic spiritual life.
But if you look closely at Philippians 2:13, Paul gives the truth. He writes, “It is God who works in you, both to will and to act according to his good purpose. God is so sovereign that his purposes are accomplished both in what we choose and what we do. But it is still we who do it. Instead of divorcing God’s sovereignty from our responsibility, as a lot of people try to do, Paul stitches them tightly together here. As the commentator John Murray wrote,
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 cooperation as if God did his part and we did ours so that the conjunction or coordination of both produced the desired result. God works and we also work. But the relation is that because God works we work. All working out of salvation on our part is a the effect of God’s working in us…The more persistently active we are in working, the more persuaded we may be that all the energizing grace and power is of God.God is working in us by His Holy Spirit. The fact that we even have the desire to change and grow to be like Christ is from the Spirit. Moreover, the harder we work to change, the more we realize that any growth we experience isn’t our doing, but God’s. God will accomplish His purpose in us, but He chooses to do so through the effort we put forth toward change in response to His Holy Spirit, which is already working in us.