You don't usually hear about the Reformation in connection with this topic. But the Reformation fought and won critical battles for a right understanding of sanctification. Renewing these triumphs over sin in our own time ought to be core to our identity as evangelicals and inheritors of the legacy from our Reformation forefathers.
These days, we usually identify only two causes with the Reformation: the final authority of the Bible and the doctrine of justification without the works of the law. In fact, when the Reformation first began, it had almost nothing to do with either of those causes. The Reformation began as an argument over sanctification.
Just read the 95 theses of Martin Luther that started it all. (No, seriously, if you've never read them, go do it.) It's all right there in thesis one: The scriptural call to repentance is a call to live our whole lives as disciples of Christ, conforming everything we do to his holiness. Thesis two adds a sharp point by clarifying specifically that the scriptural call to repentance does not refer to the sacrament of penance.
Martin Luther did not write those theses to advance a position in a dry academic disputation. The local archbishop, Albert of Mainz, had taken out a huge loan to pay the Vatican to give him a cardinalship. To pay it off, Albert drove a vicious campaign of indulgence-selling. His agents barnstormed through the German countryside, telling people that the way to be godly is by doing religious works---especially donating to the church.
The people of Wittenberg were being crushed, spiritually and financially, by a legalistic understanding of sanctification. As a parish priest, Luther had to shepherd those people. His heart was breaking for them---just as his heart had been broken years earlier, in the monastery, groaning under the weight of the same spiritual oppression in another form.
Benefit of Hindsight
To be sure, the Reformation doctrines of scripture and justification were all implied in Luther's scathing attacks on the sale of indulgences. But we only see that clearly today because we have the benefit of hindsight. Statements like thesis six ("the pope cannot remit any guilt, except by declaring that it has been remitted by God and by assenting to God's remission") may seem to our ears to be positively crying out for justification without the works of the law. That is only because the later history of the Reformation gave us the ears to hear it.
For two years, Luther insisted that his radical position on sanctification did not bring him into conflict with the church's prevailing doctrines of Scripture and justification. He even dedicated his book-length defense of the 95 theses to the pope, believing in all sincerity that he did not conflict with the magisterium. Then, at the famous "Leipzig debates" in 1519, Roman theologian Johann Eck forced Luther to begin acknowledging his deeper theological premises. (Eck would later serve as lead prosecutor in Luther's trial at the Diet of Worms.)
There's a common phrase among theologians that uses classical philosophical categories of causes to describe the Reformation: the authority of the Bible was its "formal cause" while the doctrine of justification was its "material cause." I think we can get a fuller picture of the Reformation if we add sanctification as the "efficient cause."
In classical philosophy, a formal cause is a framework or pattern or structure that gives shape ("form") to things and events. The Reformation debate was ultimately shaped in the form of a debate over the Bible's authority. Hence Luther's famous speech in his defense at Worms was not about justification but about his fidelity to "Scripture and plain reason" against popes and church councils.
A material cause is the substance or essence of which a thing is made. So long as Luther confined himself to attacking pardon-sellers, he was not excommunicated by the pope or prosecuted by the emperor. Only after Luther was forced to acknowledge his breach with Rome over justification did the Reformation truly become a radical conflict. Justification was the essence of the dispute.
An efficient cause is the force or power behind a thing; it's what makes change and motion happen. Luther turned the world upside down; the extraordinary, far-reaching reforms adopted by Rome in the Counter-Reformation are a testament to the mortal threat Luther represented. All that motion and change must have had a truly titanic efficient cause.
The Reformation was powered by a transformation in millions upon millions of people's understanding of what it means to live a godly life. Peasants and lords chose Luther over Rome, even at the cost of their lives, because the gospel message liberated them from bondage to legalistic ideas of sanctification. And the Reformation helped invent modern civilization by teaching people that making the world a better place is everyone's responsibility, and it's a 24/7 job.
By the way, there's one more classical category of causation: the final cause. This is the purpose toward which people act, the state of fulfillment things are trying to reach. And what was the final cause of the Reformation? God's glory, of course.