John Piper on Romans 13:
And so the great question that Paul is dealing with now in Romans 12–16 is what does life look like for people who know that by faith alone all their sins are forgiven, and all their condemnation is removed, and all of God’s righteousness in Christ has become their righteousness—what does life look like? How do you live the Christian life? What do you pursue? What do you focus on?
Two Different Answers
Do you say, “Now I am forgiven by faith alone, and now I have the imputed righteousness of Christ by faith alone, and now I have the Holy Spirit within me by faith alone, so now I will go back to the law—the ten commandments, and whatever other commandments there are (Romans 13:9)—and I will focus my new God-given ability on the these commandments and fulfill them”?
No. I don’t think that is the way Paul guides us. I think he wants to speak rather like this: “Now I am forgiven by faith alone, and now I have the imputed righteousness of Christ by faith alone, and now I have the Holy Spirit within me by faith alone, so now I will continue to make my focus Jesus Christ every day, and I will look to him for everything my soul craves. And from my union with Christ, nurtured hour by hour by focusing on Christ as my great Savior and mighty Lord and infinite Treasure, I will love people. Christ will be my focus, love will be my fruit.”
Knowing God through His Word ... Day by Day 
I just learned about that the Holy Sprit lives within my heart, within me as a person who sins and lives life troubled with thoughts of temptation. I am so happy that JESUS fights for me on the inside. Blessings!