In his book, By Faith, Not By Sight, Richard Gaffin includes a very helpful section dealing with the doctrine of sanctification.
There he reminds the reader that sanctification is every bit as much a part of the great salvation that all believers have in Christ as justification is. Sanctification, strictly speaking, should not be spoken of as if it were our part, while justification is God's part. Sanctification is much more than just our response to the grace of God in the gospel. Gaffin writes,
"Sanctification, first of all and ultimately, is not a matter of what we do, but of what God does. As the best in the Reformation tradition recognizes, it, no less than our justification, is a work of his grace." (p.86)
That is also the teaching of the Westminster Shorter Catechism, which defines sanctification as "the work of God's free grace." It is, first and foremost, God's work - His ongoing work of renewal in the lives of believers, by which we are "enabled more and more to die unto sin, and live unto righteousness" (Q.35). It is not only justification that is by God's free grace, but sanctification as well!
That, of course, does not mean that sanctification doesn't involve activity or effort on our part, but that activity or effort should not be seen as meritorious; it does not earn anything. Sanctification is a gracious blessing from God, not a bargaining chip. It is what the Apostle Paul was referring to in Philippians 2:12-13 where he wrote,
"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." (ESV)
Sanctification is our 'working out' what God Himself has 'worked in,' which includes not just the doing or working for His good pleasure, but even our willingness or desire to do so in the first place! We are sanctified by the free grace of God.