In his writings, Paul uses these terms—faith, grace, and justification—interchangeably. He uses one word here, another there, depending on which nuance he wants to bring to the fore, so that we get a complete picture of what is happening. Here, he is talking about faith, and within the subject of justification, he says, "No, faith in the blood of Jesus Christ establishes the law!" not "does away with" it. Faith in no way invalidates God's law. None of it!
Notice that your Bible very likely reads "the law." However, it does not say that in the Greek; the definite article does not precede "law" either time it appears in this verse. The Interlinear Bible, which is a literal translation, reads: "Law then do we nullify through faith? Not let it be! But law do we establish." Establish means "cause to stand, confirm."
One might argue, "What difference does the lack of an article make?" In this case, if it read "the law," Paul would have been referring to either the entire Pentateuch or to a specific law. But writing it as he did, he means law in general as a legal argument. Any law! Man's law, God's law, the Ten Commandments, the sacrifices—everything is included under that blanket statement. He says, "Faith establishes law." It remains for other passages to tell us about a specific law or body of laws that might be set aside. So, then, faith—used here in connection with grace and justification—establishes law. It does NOT do away with it; such an interpretation is the exact opposite of what is written!
When a person is justified, it is for the very reason that he is out of alignment with what he is being measured against. So after justification, the standard is not just thrown away! Indeed, the standard becomes more important than ever because we do not want to get out of alignment ever again. We need the law's guidance to help us in what we must do and to warn us when we are veering from the way.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 4)
We are justified through faith in the sacrifice of Jesus Christ. He is the payment for our sins, thus freeing us from sin's penalty, and at the same time, God accounts—or imputes—Christ's righteousness to us. The righteousness that enabled Him to be the perfect sacrifice is accounted as if it is ours! This then makes it possible for us to have access into the presence of the holy God.
But this does not do away with law. It establishes it! It places the law in its rightful position in our understanding of what God is working out in our lives.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 29)
Other Forerunner Commentary entries containing Romans 3:31: