R. Scott Clark is very helpful here:
“The law never gives us the ability to do what it commands. Only God the Spirit does that, through the word of the gospel.”
Here’s the quote in context:
R. Scott Clark answers a question on the Heidelblog a few years ago regarding the agreements and differences between Reformed and Lutheran Orthodoxy. In writing about the differences and doctrines that the two hold in common, Clark mentions something important and helpful citing from the Belgic Confession on Sanctification and a couple of books covering Sanctification. Quoting now:
I think this agrees with the Reformed doctrine of progressive sanctification. In the Belgic Confession (Art 24) we confess:
We believe that this true faith, produced in man by the hearing of God’s Word and by the work of the Holy Spirit, regenerates him and makes him a “new man,” causing him to live the “new life” and freeing him from the slavery of sin.
Therefore, far from making people cold toward living in a pious and holy way, this justifying faith, quite to the contrary, so works within them that apart from it they will never do a thing out of love for God but only out of love for themselves and fear of being condemned. So then, it is impossible for this holy faith to be unfruitful in a human being, seeing that we do not speak of an empty faith but of what Scripture calls “faith working through love,” which leads a man to do by himself the works that God has commanded in his Word.
These works, proceeding from the good root of faith, are good and acceptable to God, since they are all sanctified by his grace. Yet they do not count toward our justification– for by faith in Christ we are justified, even before we do good works. Otherwise they could not be good, any more than the fruit of a tree could be good if the tree is not good in the first place.
So then, we do good works, but nor for merit– for what would we merit? Rather, we are indebted to God for the good works we do, and not he to us, since it is he who “works in us both to will and do according to his good pleasure”60– thus keeping in mind what is written: “When you have done all that is commanded you, then you shall say, ‘We are unworthy servants; we have done what it was our duty to do.’ “
Yet we do not wish to deny that God rewards good works– but it is by his grace that he crowns his gifts. Moreover, although we do good works we do not base our salvation on them; for we cannot do any work that is not defiled by our flesh and also worthy of punishment. And even if we could point to one, memory of a single sin is enough for God to reject that work.
So we would always be in doubt, tossed back and forth without any certainty, and our poor consciences would be tormented constantly if they did not rest on the merit of the suffering and death of our Savior.
In the brief essay to which I referred above, David doesn’t say which “Protestants” he has in mind but the Reformed churches agree that believers are no longer under the curse of the law. We agree that we are not sanctified by the law, but we confess that the law is the norm for our sanctification. As Walter Marshall explained in The Gospel Mystery of Sanctification, and as Mike Horton has recently explained in The Gospel-Driven Life, the gospel is the power of the Christian life. The law never gives us the ability to do what it commands. Only God the Spirit does that, through the word of the gospel. Nevertheless, as the Epitome says, we are not idle. Sanctification is by grace alone, but that grace is operative in us and through us and enables us to cooperate toward Christlikeness in this life.