Answer:
This is an excellent question and one that was asked early on in the Christian era. It's a logical question coming right out of the gospel's emphasis on grace based salvation. It's so logical and natural an assumption to make that Paul asked the rhetorical question himself when presenting the grace alone, no earning message of Christ in Romans. He says: Rom 6:1-2 "What shall we say, then? Shall we go on sinning so that grace may increase?" And the answer is, "By no means!" (6:2).
You've clearly heard the "by no means" part - you know that we Christians are still called to be good, but WHY? Why try to be good if we can never be good enough for God and if God imputes his goodness (righteousness) to us as a gift (Rom 6:23)? Doesn't trying to be good put us right back where we were, trying to earn salvation and earn God's love by our good deeds - something Paul already said was impossible to do (Rom 3:20, Gal 2:21)?
The WHY and the WAY we still try to be good are found in Paul's argument in Romans 6-8 directly following his explanation of grace. Make sure you read it. Grace is unearned, but it is accessed by a condition: simple faith, and repentance. FAITH means we trust God to do it, we leave behind our fears of punishment and trust in the merciful heart of God based on the debt payment of the Cross on our behalf. REPENTANCE means we "die" to an old way of life lived apart from God. It's a change of MIND before it's a change of behavior. We die to everything that separated us from God - INCLUDING the pride that thought we could earn our way into God's favor by being good enough. We die to unrighteousness AND to self righteousness.
This "death" means of course that I don't arrogantly carry on into the Kingdom in the same way of life I lived before. Entry into the Kingdom is free, but the "narrow door" into it which Jesus talked about, is this dying to self. "Blessed are the poor in spirit". "Blessed are they who mourn". "Blessed are the pure in heart for they shall see God." Clearly then, people who exhibit this kind of broken humility would never think of carrying on in the same sin uninterrupted upon hearing that God has decided to welcome us into his Kingdom. It's unthinkable.
Now, a broken humility does not imply that I'm suddenly perfect. It just means that I've had a change of mind (repentance) about sin, both my unrighteousness and my self righteousness. I'm newly motivated. And that's the crux of it right there. Before Christ i try to please God out of an anxious sense of trying to make God love me. After Christ, i try to please God out of an overwhelming sense of love and gratitude for his unearned favor, knowing i can't make him love me more, but now I want to love HIM more!
Here's Paul putting it bluntly: Rom 7:6 "But now, by dying to what once bound us, we have been released from the law so that we serve in the new way of the Spirit, and not in the old way of the written code."
Do we still serve? Yes! But in a 'new way', he says. It's the way of the Spirit, which is by God's power and grace, rather than the old way of the written code, which is the way of earning. We know we can't do enough to earn anything... but we are so overwhelmingly loved that our response is obedience, and a desire to please the God who sought us and bought us with the Life of his Son.
So there are two people trying to be good, one is a Christian and the other is not; one is trying to put God in his debt so that God owes him something (blessing, heaven, superior status over his neighbor etc), the other has had his unpayable debt paid and rejoices at the chance to please the God to whom he owes his life; the one is never certain where he stands, one day he's superior the next day he's guilt ridden, the other is adopted into God's family and so he's never uncertain where he stands because nothing can condemn him if God himself has decided to justify him.
Will we be perfect? Of course not. But beyond the new motive we have to please God, we have new resources to live this life of gratitude to God for his grace. The Holy Spirit gives us power to walk the way Christ walked if we train our minds on him because He is our moment to moment Partner. Rom 8:6: "The mind of sinful man is death, but the mind controlled by the Spirit is life and peace." In a living, constant conversation with Him, we will find the strength to please God more successfully than we ever did when we were on the 'earning' plan.
