So then, my beloved, just as you have always obeyed, not as in my presence only, but now much more in my absence, work out your salvation with fear and trembling; for it is God who is at work in you, both to will and to work for His good pleasure (Philippians 2:12-13 NASB).
The matter of behaviour is ours, not God's. God does not make our character; character is formed by the reaction of our inner disposition to outer things through our nervous system. God does what we cannot do: He alters the mainspring and plants in us a totally new disposition; then begins our work, we must work out what God works in. The practising is ours, not God's. We have to bring the mechanism of body and brain into line by habit and make it a strong ally of the grace of God (Oswald Chambers, "The Moral Foundation of Life: A Series of Talks on the Ethical Principles of the Christian Life.").
We need clarity regarding grace, mercy and reformation of life. Sinners need to know about God's grace and mercy. The reason is simple. No amount of reform resolves the problem of sin. That's hard for us to grasp because the sense of justice demands that we somehow make up for wrongs done. It won't ever happen. You can't out-good the evil in your life. God's law of sin and death says, "If you sin, you die." There is no provision for overcoming sin by the practice of good. Forgiveness and salvation are the result of God's grace and mercy. By grace we get what we don't deserve. By mercy we don't get what we do deserve. All that comes from God, and it's what makes the gospel of Jesus good news.
But don't make the mistake of thinking that there is no room for reformation. In the gospel, there is also the call for confession of sin, repentance, and reformation of life. God's forgiveness and salvation may be instantaneous, but there remains a lifetime of "working out" our salvation. That doesn't mean that we work to earn salvation. It means that we see it through to the end. It means we don't give up on the effort to grow in God's grace and respond to God's mercy by distancing ourselves from all that created distance between us and God.
Our behavior must become, as Oswald Chambers might say it, a "strong ally of the grace of God." That's an excellent way to put it. There are at least two ways in which Christians fail in this regard. The first is thinking that God's grace and mercy are all there is to forgiveness and salvation. This just isn't so. God saves by His mercy and grace, but He also intends to change us. The second mistake is to think that reformation of life isn't all that important. Paul would ask, "How shall we who died to sin still live in it?" (Romans 6:2).
There is another way in which Christians miss the intended effects of the gospel. We can ignore or deny the fact that we all have a long way to go to become the person God wants us to be. We love to classify sins into big ones and little ones, and then further classify our personal sins as little. We fool ourselves into thinking there just isn't much to reform. This is a bad mistake.
Accept the salvation of God rooted in His grace and mercy. Celebrate the work of Jesus on our behalf, doing for us what we can never do. Be glad that you are saved, not on the basis of your merit or goodness, but on that of Jesus Christ. Now work out that salvation by being transformed into something you could never be without that grace and mercy. When you fail, God's grace and mercy will be there to pick you up, dust you off, and get you going again, so that you can stay at the task of reformation.