Romans 3:31

Do we, then, nullify the law by this faith? Not at all! Rather, we uphold the law.

(v31, NIV)

A legitimate question would be to ask if the law is null because of faith. If we are justified by faith and boasting is excluded, what's the point of the law? However, even though we are justified by faith, it doesn't mean the law is nullified. We don't set aside the observance of the law when we pick up faith. Instead, we uphold the law. If you are declared righteous on the basis of faith, you don't go and live in sin. There is a sort of connection that leads from faith to the observance of the law. If you have faith, you will keep the law. This doesn't mean that those who are justified are perfect and never sin. However, the point is that faith and the law are not opposites and they actually work together quite nicely.