WLC Q. 97. What special use is there of the moral law to the regenerate?
A. Although they that are regenerate, and believe in Christ, be delivered from the moral law as a covenant of works, so as thereby they are neither justified nor condemned; yet besides the general uses thereof common to them with all men [see Q. 95 below], it is of special use, to show them how much they are bound to Christ for his fulfilling it, and enduring the curse thereof in their stead, and for their good; and thereby to provoke them to more thankfulness, and to express the same in their greater care to conform themselves thereunto as the rule of their obedience.
WLC Q. 95. Of what use is the moral law to all men?
A. The moral law is of use to all men, to inform them of the holy nature and will of God, and of their duty, binding them to walk accordingly; to convince them of their disability to keep it, and of the sinful pollution of their nature, hearts, and lives: to humble them in the sense of their sin and misery, and thereby help them to a clearer sight of the need they have of Christ, and of the perfection of his obedience.So the moral law points the regenerate to Christ and his fulfilling of that law as a covenant of works for us (which we couldn't fulfil and still can't) and that he endured the curse of the law in our place (we are miserable offenders). And all of this he alone accomplished for our good. And it is this good news that increases and provokes more and more thankfulness in us as we grow in the blessedness of God’s grace in which we stand. That thankfulness finds its expression in our grateful duty as we endeavor to walk in conformity to Christ’s moral law, it being the guide or rule of our obedience.
Regarding sanctification, it seems to be more a function of God’s grace than his law:
WSC Q. 35. What is sanctification?Certainly the law is used by God, through both preaching and His written Word, yet it is Christ himself who has been given to us for both justification and sanctification and, as the earlier quotes make clear, even our whole salvation. John Calvin responding to Sadoleto:
A. Sanctification is the work of God’s free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness.
… We deny that good works have any share in justification, but we claim full authority for them in the lives of the righteous. For if he who has obtained justification possesses Christ, and at the same time, Christ never is where His Spirit is not, it is obvious that gratuitous righteousness is necessarily connected with regeneration. Therefore, if you would duly understand how inseparable faith and works are, look to Christ, who, as the Apostle teaches (1 Cor. 1:30) has been given to us for justification and for sanctification.Wherever, therefore, that righteousness of faith, which we maintain to be gratuitous, is, there too Christ is, and where Christ is, there too is the Spirit of holiness, who regenerates the soul to newness of life.