how do law and faith relate?

"...faith alone makes someone just and fulfills the law; faith it is that brings the Holy Spirit through the merits of Christ. The Spirit, in turn, renders the heart glad and free, as the law demands. Then good works proceed from faith itself. That is what Paul means in chapter 3 when, after he has thrown out the works of the law, he sounds as though the wants to abolish the law by faith. No, he says, we uphold the law through faith, i.e. we fulfill it through faith." - Martin Luther, Intro to Romans