What is the duty of grace? For Paul, it's a simple one word answer: Love.
In response to grace we show fealty to God by loving each other.
Romans 12 is a beautiful example of this. After eleven chapters discussing grace and faith, Paul turns to the duties of grace, how we offer our lives as a "living sacrifice" in fealty to the God who gives us grace:
Just as our bodies have many parts and each part has a special function, so it is with Christ’s body. We are many parts of one body, and we all belong to each other...Don’t just pretend to love others. Really love them. Hate what is wrong. Hold tightly to what is good. Love each other with genuine affection, and take delight in honoring each other. Never be lazy, but work hard and serve the Lord enthusiastically. Rejoice in our confident hope. Be patient in trouble, and keep on praying. When God’s people are in need, be ready to help them. Always be eager to practice hospitality.In the very next chapter, Paul gives a succinct summary of the duty of grace:
Bless those who persecute you. Don’t curse them; pray that God will bless them. Be happy with those who are happy, and weep with those who weep. Live in harmony with each other. Don’t be too proud to enjoy the company of ordinary people. And don’t think you know it all!
Never pay back evil with more evil. Do things in such a way that everyone can see you are honorable. Do all that you can to live in peace with everyone.
Dear friends, never take revenge. Leave that to the righteous anger of God. For the Scriptures say,
“I will take revenge; I will pay them back,” says the Lord.
Instead,
“If your enemies are hungry, feed them.
If they are thirsty, give them something to drink.
In doing this, you will heap
burning coals of shame on their heads.”
Don’t let evil conquer you, but conquer evil by doing good.
Owe nothing to anyone—except for your obligation to love one another. If you love your neighbor, you will fulfill the requirements of God’s law. For the commandments say, “You must not commit adultery. You must not murder. You must not steal. You must not covet.” These—and other such commandments—are summed up in this one commandment: “Love your neighbor as yourself.” Love does no wrong to others, so love fulfills the requirements of God’s law.The examples of this are everywhere in Paul. In 1 Corinthians we have Chapter 13 telling us about the duty of grace: "Love is patient, love is kind."
We find it in Philippians 2: "Do nothing out of selfish ambition or vain conceit. Rather, in humility value others above yourselves, not looking to your own interests but each of you to the interests of the others. In your relationships with one another, have the same mindset as Christ Jesus: Who, being in very nature God, did not consider equality with God something to be used to his own advantage; rather, he made himself nothing by taking the very nature of a servant."
So let's not have a grim puritanical imagination when we hear the words duty and obligation.
The fealty grace demands is love. Because of grace, as Paul says, "we belong to each other." And so its our duty to love each other with "genuine affection" and "delight."
This is the duty of grace.