Calvin’s catechism, Instruction In Faith, of 1537 offers a fantastic section (17) on what is most often referred to as the 3rd use of the Law (Sanctification):
Just as Christ by means of his righteousness intercedes for us with the Father in order that (he being as our guarantor) we may be considered as righteous, so by making us participants in his spirit, he sanctifies us unto all purity and innocence. For the spirit of the Lord has reposed on Christ without measure – the spirit (I say) of wisdom, of intelligence, of counsel, of strength, of knowledge and reverential fear of the Lord – in order that we all may draw from his fullness and receive grace through the grace that has been given to Christ. As a result, those who boast of having the faith of Christ and are completely destitute of sanctification by his spirit deceive themselves. For the Scripture teaches that Christ has been made for us not only righteousness but also sanctification. Hence, we cannot receive through faith his righteousness without embracing at the same time that sanctification, because the Lord is one same alliance, which he has made with us in Christ, promises that he will be propitious toward our iniquities and will write his Law in our hearts (Jer. 31:33; Heb. 8:10; 10:16).
Observance of the Law, therefore, is not a work that our power can accomplish, but it is a work of a spiritual power. Through this spiritual power it is brought about that our hearts are cleansed from their corruption and are softened to obey unto righteousness. Now the function of the Law is for Christians quite different from what it may be without faith; for, when and where the Lord has engraved in our hearts the love for his righteousness, the external teaching of the Law (which before was only charging us with weakness and transgression) is now a lamp to guide our feet, to the end that we may not deviate from the right path. It is now our wisdom through which we are formed, instructed, and encouraged to all integrity; it is our discipline which does not suffer us to be dissolute through evil licentiousness.
(By: Nick Kennicott)